Nitrate reductases from red algae, compositions and methods of use thereof

ABSTRACT

The NR enzymes described herein were discovered in the red algae of  Porphyra perforata  (PpNR) and  Porphyra yezoensis  (PyNR). The present invention provides methods and compositions relating to altering NR activity, nitrogen utilization and/or uptake in plants. The invention relates to a method for the production of plants with maintained or increased yield under low nitrogen fertility. The invention provides isolated nitrate reductase (NR) nucleic acids and their encoded proteins. The invention further provides recombinant expression cassettes, host cells, and transgenic plants. Plants transformed with nucleotide sequences encoding the NR enzyme show improved properties, for example, increased yield and growth.

CROSS REFERENCE

This divisional application claims benefit of U.S. patent applicationSer. No. 12/900,600, filed Oct. 8, 2010 which claims benefit of U.S.patent application Ser. No. 12/138,477, filed Jun. 13, 2008, now U.S.Pat. No. 7,834,245 issued Nov. 6, 2010, which claims the benefit U.S.Provisional Application Ser. No. 60/944,343, filed Jun. 15, 2007, all ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The domestication of many plants has correlated with dramatic increasesin yield. The identification of specific genes responsible for thedramatic differences in yield, in domesticated plants, has become animportant focus of agricultural research.

One group of genes effecting yield are the nitrate reductase genes.These genes have utility for improving the use of nitrogen in cropplants, especially maize. The genes can be used to alter the geneticcomposition of the plants rendering them more productive with currentfertilizer application standards or maintaining their productive rateswith significantly reduced fertilizer input. Increased nitrogen useefficiency can result from enhanced uptake and assimilation of nitrogenfertilizer and/or the subsequent remobilization and reutilization ofaccumulated nitrogen reserves. Plants containing these genes cantherefore be used for the enhancement of yield. Improving the nitrogenuse efficiency in corn would increase corn harvestable yield per unit ofinput nitrogen fertilizer, both in developing nations where access tonitrogen fertilizer is limited and in developed nations were the levelof nitrogen use remains high. Nitrogen utilization improvement alsoallows decreases in on-farm input costs, decreased use and dependence onthe non-renewable energy sources required for nitrogen fertilizerproduction and decreases the environmental impact of nitrogen fertilizermanufacturing and agricultural use.

Many efforts have recently been made to improve nitrogen efficiency ofcrop plants through overexpression of nitrate reductase (NR) in plants.One group has applied expression of the Nicotiana plumbaginifolia andArabidopsis thaliana NR cDNA or gene under control of differentpromoters such as ³⁵S CaMV (Nicotiana plumbaginifolia) Ferrario, et al.,(1995) Planta 196:288-294 and Lhcb1*3::Nia1*2 (Arabidopsis thaliana)Nejidat, et al., (1997) Plant Science 130:41-49. (References). Althoughincreases in NR expression levels up to 2-5-fold were detected with the³⁵S CaMV::Nia-2 gene in Nicotiana plumbaginifolia plants (Foyer, et al.,(1994) Plant Physiol. 104:171-178), no improved nitrogen efficiency wasobserved. All these attempts to over express this enzyme have notresulted in improved growth under lower nitrogen fertility.

Therefore, despite several attempts to improve NR efficiency, nosatisfactory composition or method has been provided that leads to animprovement of growth, productivity and/or yield for agricultural cropplants. For these and other reasons, there is a need for the presentinvention.

BRIEF SUMMARY OF THE INVENTION

The present invention provides polynucleotides, related polypeptides andall conservatively modified variants of the present NR sequences. Theinvention provides sequences for the NR genes.

The present invention presents methods to alter the genetic compositionof crop plants, especially maize, so that such crops can be moreproductive with current fertilizer applications and/or as productivewith significantly reduced fertilizer input. The utility of this classof invention is then both yield enhancement and reduced fertilizer costswith corresponding reduced impact to the environment.

Therefore, in one aspect, the present invention relates to an isolatednucleic acid comprising an isolated polynucleotide sequence encoding anNR gene. One embodiment of the invention is an isolated polynucleotidecomprising a nucleotide sequence selected from the group consisting of:(a) the nucleotide sequence comprising SEQ ID NO: 1, 2, 3, 4, 5 or 6 and(c) the nucleotide sequence comprising at least 70% sequence identity toSEQ ID NO: 1, 2, 3, 4, 5 or 6, wherein said polynucleotide encodes apolypeptide having increased NR activity.

Compositions of the invention include an isolated polypeptide comprisingan amino acid sequence selected from the group consisting of: (a) theamino acid sequence comprising SEQ ID NO: 7, 8, 9, 10, 11 or 12 and (b)the amino acid sequence comprising at least 70% sequence identity to SEQID NO: 7, 8, 9, 10, 11 or 12, wherein said polypeptide has increased NRactivity.

In another aspect, the present invention relates to a recombinantexpression cassette comprising a nucleic acid as described.Additionally, the present invention relates to a vector containing therecombinant expression cassette. Further, the vector containing therecombinant expression cassette can facilitate the transcription andtranslation of the nucleic acid in a host cell. The present inventionalso relates to the host cells able to express the polynucleotide of thepresent invention. A number of host cells could be used, such as but notlimited to, microbial, mammalian, plant or insect.

In yet another embodiment, the present invention is directed to atransgenic plant or plant cells, containing the nucleic acids of thepresent invention. Preferred plants containing the polynucleotides ofthe present invention include but are not limited to maize, soybean,sunflower, sorghum, canola, wheat, alfalfa, cotton, rice, barley,tomato, and millet. In another embodiment, the transgenic plant is amaize plant or plant cells. Another embodiment is the transgenic seedsfrom the transgenic NR polypeptide of the invention operably linked to apromoter that drives expression in the plant. The plants of theinvention can have altered NR as compared to a control plant. In someplants, the NR is altered in a vegetative tissue, a reproductive tissueor a vegetative tissue and a reproductive tissue. Plants of theinvention can have at least one of the following phenotypes includingbut not limited to: increased root mass, increased root length,increased leaf size, increased ear size, increased seed size, increasedendosperm size and increased biomass

Another embodiment of the invention would be plants that have beengenetically modified at a genomic locus, wherein the genomic locusencodes a NR polypeptide of the invention.

Methods for increasing the activity of a NR polypeptide in a plant areprovided. The method can comprise introducing into the plant an NRpolynucleotide of the invention.

Methods for reducing or eliminating the level of NR polypeptide in theplant are provided. The level or activity of the polypeptide could alsobe reduced or eliminated in specific tissues, causing alteration inplant growth, growth rate or nitrogen utilization efficiency (NUE).Reducing the level and/or activity of the NR polypeptide may lead tosmaller stature or slower growth of plants.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood from the following detaileddescription and the accompanying drawing and Sequence Listing which forma part of this application.

FIG. 1. Peptide sequence comparison between Porphyra perforata (PpNR—SEQID NO: 5) and P. yezoensis (PyNR—SEQ ID NO: 10). PpNR and PyNR areapproximately 94% identical. The identical amino acid residues are inbold and similar ones are underlined.

FIG. 2. Yeast nitrate transporter YNT1 was cloned from Pichia angustabased on the published sequence (GenBank Accession Number Z69783). YNT1driven by a constitutive promoter glyceraldehyde-3-phosphatedehydrogenase (GAP) from P. pastoris with histidine auxotroph selectionmarker (p3.5GAP-YNT1) was integrated into His4 locus of P. pastorisstrain KM71 (Invitrogen) using Pichia EasyComp transformation kit(Invitrogen). The recombinant strains were confirmed to carry the YNT1expression cassette by PCR. The KM71-containing YNT1 line wasre-transformed with nitrate reductase gene PPNR from Porphyra perforatedriven by GAP promoter with Zeocin selection marker (pAOXGAP-PPNR) whichintegrated into AOX1 locus of P. pastoris genome. Nitrate reductaseenzyme activity was assayed in vivo. Four transformants and KM71 wildtype were cultured in rich media (YPD) at 30° C. for overnight. Yeastcells were collected and washed with water twice then re-suspended in 20uM MOPS, pH6.5 and 1% glucose containing 5 mM NaNO₃. After 1 hourincubation at 30° C., the supernatant was collected for nitrite assaywith 1% Sulfanilamide, 0.01% N-(1-Naphthyl) ethylene-diaminedihydrochloride and 15% (v/v) H₃PO₄. Strong NR activity was detected intransformants carrying PPNR comparing to KM71 wild type strain.

FIG. 3. The KM71-containing YNT1 line (see, FIG. 2) was re-transformedwith nitrate reductase gene PYNR from Porphyra yezoensis driven by GAPpromoter with Zeocin selection marker (pAOXGAP-PYNR) which integratedinto AOX1 locus of P. pastoris genome. In this case, the nitratereductase enzyme activity from two transformants and KM71 wild type wasassayed in vivo as before. The NR activity was detected from thetransformants carrying PYNR comparing to KM71 wild type strain. However,the PYNR activity is much weaker then PPNR in P. pastoris.

FIG. 4. Yeast nitrate reductase YNR1 was cloned from Pichia angustabased on the published sequence (GenBank Accession Number Z49110). TheKM71-containing YNT1 line was re-transformed with nitrate reductase geneYNR1 driven by GAP promoter with Zeocin selection marker (pAOXGAP-YNR1)which integrated into AOX1 locus of P. pastoris genome. The transformantwith the best Vmax was used for kinetic study.

Maize nitrate reductase ZmNR was cloned from maize B73 (assembled fromtwo truncated ESTs) based on the published genomic sequence (GenBankAccession Number AF153448). The cloned ZmNR has three different aminoacid residues compared to AF153448. The KM71-containing YNT1 line wasre-transformed with nitrate reductase gene ZMNR driven by GAP promoterwith Zeocin selection marker (pAOXGAP-ZMNR) which integrated into AOX1locus of P. pastoris genome. The transformant with the best Vmax wasused for kinetic study.

The transformants carrying YNT1/YNR1, YNT1/ZMNR or YNT1/PPNR and KM71wild type were cultured in rich media (YPD) at 30° C. for overnight.Yeast cells were collected and washed with water twice then re-suspendedin 20 μM MOPS, pH6.5 or 20 μM MES, pH5.5 or 20 μM Tris, pH7.5 with 1%glucose containing 24 different concentrations of NaNO₃ from 0 up to 30mM (0, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18, 0.2, 0.3,0.4, 0.6, 0.8, 1, 2, 4, 8, 10, 15, 20, 25 and 30 mM). The reducednitrite was assayed as mentioned above. The Km was estimated as thesubstrate concentration at ½ Vmax. The Km and preferred pH (with thebest Vmax) of YNR1, ZMNR and PPNR was summarized in the table. Pleasenote that although the diagram shows YNR1 as the lowest line on thechart, only one tenth the volume of YNR1 material was used in theexperiment, therefore the YNR1 had the best Vmax in the experiment.

FIG. 5. The constructs of PPNR and PYNR driven by UBI promoter and/ormaize nitrate reductase ZMNR promoter were made. The resultingconstructs, PHP27800, PHP27801, PHP28335 and PHP28334 were used formaize transformation. Transgenic maize segregating 1:1 for UBI:Porphyraperforata NR was grown to anthesis in hydroponics medium maintained at 1mM KNO₃ as the sole nitrogen source. Plants containing the transgenewere identified during growth. All plants were harvested shortly afteranthesis. The ear and the remaining plant were separated, dried in aforced air oven at 70° C. for 72 hr and weighed. Mean ear dry weight ofeach event transgenic plants were compared to the ear dry weight of thecorresponding nulls. Similar comparisons were made between transgenicmean plant dry weight and the corresponding event nulls. Comparisonsmarked with * are statistically significant (p>t=0.1). This figure is agraphical representation of the data described in Example 10.

BRIEF DESCRIPTION OF THE SEQUENCES

The application provides details of NR sequences as shown in Table 1below.

TABLE 1 SEQ ID Polynucleotide (pnt) NO: or polypeptide (ppt) LengthIdentification 1 pnt 2865 PpNR. 2 pnt 2865 Codon optimizedpolynucleotide sequence encoding PpNR S561Ala 3 pnt 2865 Codon optimizedpolynucleotide sequence encoding PpNR S561Asp 4 pnt 2871 PyNR 5 pnt 2871Codon optimized polynucleotide sequence encoding PyNR S563Ala 6 pnt 2871Codon optimized polynucleotide sequence encoding PyNR S563Asp 7 ppt 954PpNR. 8 ppt 954 PpNR S561Ala - encoded by codon optimized polynucleotide9 ppt 954 PpNR S561Asp - encoded by codon optimized polynucleotide 10ppt 954 PyNR 11 ppt 956 PyNR S563Ala - encoded by codon optimizedpolynucleotide 12 ppt 956 PyNR S563Asp - encoded by codon optimizedpolynucleotide 13 pnt 26 PYNR sense primer 14 pnt 20 PYNR anti-senseprimer 15 pnt 20 PpNR walking primer 16 pnt 20 PpNR walking primer 17pnt 20 PpNR walking primer 18 pnt 25 PpNR walking primer 19 pnt 24 PpNRwalking primer 20 pnt 25 PpNR walking primer 21 pnt 21 ORF PpNR senseprimer 22 pnt 21 ORF PpNR anti-sense primer 23 pnt 19 ORF PyNR senseprimer 24 pnt 22 ORF PyNR anti-sense primer

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Unless mentioned otherwise, thetechniques employed or contemplated herein are standard methodologieswell known to one of ordinary skill in the art. The materials, methodsand examples are illustrative only and not limiting. The following ispresented by way of illustration and is not intended to limit the scopeof the invention.

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

Overview

Nitrate is the most important source of nitrogen for higher plants.Nitrate is absorbed by the roots, transported to various tissues of theplant and then reduced to aqueous ammonia in two steps. The first steprequires the enzyme nitrate reductase (NR), which catalyzes thereduction of nitrate to nitrite in the cytoplasm. In a second step, thenitrite is then reduced in the chloroplast by nitrite reductase. Thereduction of nitrate is considered to be limiting step in nitratemetabolism in plants.

Plant nitrate reductases are regulated at both transcriptional andpost-translational levels. Several environmental factors such as lightor nitrate regulate NR gene expression at transcriptional level. NRactivity is also regulated at post-translational level by light. NRbecomes inactive/active in response to dark/light byphosphorylation/dephosphorylation at putative Ser residues. The inactiveform of NR binds to 14-3-3 protein in the dark. The putative regulationsites include N-terminal region (Plant Cell (1995) 7:611-621), hinge 1(Plant J. (2003) 35:566-573) and other regions.

The present invention relates to the discovery of novel NRs from redalgae (division of Rhodophyta) of Porphyra perforate (PpNR) and Porphyrayezoensis (PyNR). As described herein, the inventors have identified twonovel NR cDNAs in red algae that share approximately 94% amino acidconsensus with respect to one another but only 52% amino acid identityto a maize homolog of NR from B73 (Genbank Accession Number AF153448).The algal NR polynucleotides of this invention are 2865 bp (PpNR) and2871 bp (PyNR), nucleotides in length encoding polypeptides withcalculated molecular weight of 104.9 KDa (PpNR) and 105.2 KDa (PyNR).Also contemplated are variants of these sequences. For example, mutatingthe serine residue in a putative phosphorylation motif R/K-S/T-X-pS-X-P(J of Experimental Botany (2004) 55:1275-1282) at position 561 to analanine or aspartic acid of the PpNR or an alanine or aspartic acid atposition 563 of the PyNR sequence is believed to increase and maintainhigh level of active form of nitrate reductase in transgenic plants tofacilitate nitrate assimilation.

Red algae (division of Rhodophyta) of Porphyra perforate (PpNR) andPorphyra yezoensis (PyNR) grow in concentrations of nitrate that areapproximately one hundred times lower than the nitrate concentrations inwhich plants are capable of growing. These red algae have more efficientnitrate reductase enzymes that saturate at lower substrateconcentrations than higher plant nitrate reductase. In particular,Porphyra perforate and Porphyra yezoensis have been reported in theliterature to have a NR enzyme with a low Km for nitrate (30-65 μM).This is in contrast to a maize NR from B73 which has a Km for nitratereductase of 300 μM. In vivo kinetic measurements made of PpNR and PyNRexpressed in Pichia pastoris show this enzyme has a Km for nitrate of30-50 μM compared to maize nitrate reductase Km of 200 μM. Withoutwishing to be bound by this theory, it is believed that expressing moreefficient NRs that saturate at lower concentrations of substrate thanthe higher plant NR enzymes will be more efficient in nitrogenreduction. Modulation of the NRs of the present invention would providea mechanism for manipulating a plant's nitrogen utilization efficiency(NUE). Accordingly, the present invention provides methods,polynucleotides and polypeptides for the production of plants withimproved or maintained yield under limited nitrogen supply. In oneaspect, the methods include introducing into a plant cell, plant tissueor plant one or more polynucleotides encoding NR polypeptides having theenzymatic activity of nitrate reductase. This may be accomplished byintroducing the nitrate reductase polynucleotides driven by aconstitutive promoter or a mesophyll cell preferred promoter into theplant nuclear genome.

Advantageously, plants expressing a NR of the present invention willprovide the customer increased revenue by lowering input costs and/orincreasing yields with a significant reduction in applied nitrogenfertilizer. Furthermore, yields may be maintained or increased in plantsexpressing a NR of the present invention even under non-favorable growthconditions, for example, where nitrogen is in limited supply.

The practice of the present invention will employ, unless otherwiseindicated, conventional techniques of botany, microbiology, tissueculture, molecular biology, chemistry, biochemistry and recombinant DNAtechnology, which are within the skill of the art. Such techniques areexplained fully in the literature. See, e.g., Langenheim and Thimann,(1982) Botany: Plant Biology and Its Relation to Human Affairs, JohnWiley; Cell Culture and Somatic Cell Genetics of Plants, vol. 1, Vasil,ed. (1984); Stanier, et al., (1986) The Microbial World, 5^(th) ed.,Prentice-Hall; Dhringra and Sinclair, (1985) Basic Plant PathologyMethods, CRC Press; Maniatis, et al., (1982) Molecular Cloning: ALaboratory Manual; DNA Cloning, vols. I and II, Glover, ed. (1985);Oligonucleotide Synthesis, Gait, ed. (1984); Nucleic Acid Hybridization,Hames and Higgins, eds. (1984); and the series Methods in Enzymology,Colowick and Kaplan, eds, Academic Press, Inc., San Diego, Calif.

Units, prefixes and symbols may be denoted in their SI accepted form.Unless otherwise indicated, nucleic acids are written left to right in5′ to 3′ orientation; amino acid sequences are written left to right inamino to carboxy orientation, respectively. Numeric ranges are inclusiveof the numbers defining the range. Amino acids may be referred to hereinby either their commonly known three letter symbols or by the one-lettersymbols recommended by the IUPAC-IUB Biochemical NomenclatureCommission. Nucleotides, likewise, may be referred to by their commonlyaccepted single-letter codes. The terms defined below are more fullydefined by reference to the specification as a whole.

DEFINITIONS

In describing the present invention, the following terms will beemployed and are intended to be defined as indicated below.

By “microbe” is meant any microorganism (including both eukaryotic andprokaryotic microorganisms), such as fungi, yeast, bacteria,actinomycetes, algae and protozoa, as well as other unicellularstructures.

By “amplified” is meant the construction of multiple copies of a nucleicacid sequence or multiple copies complementary to the nucleic acidsequence using at least one of the nucleic acid sequences as a template.Amplification systems include the polymerase chain reaction (PCR)system, ligase chain reaction (LCR) system, nucleic acid sequence basedamplification (NASBA, Cangene, Mississauga, Ontario), Q-Beta Replicasesystems, transcription-based amplification system (TAS) and stranddisplacement amplification (SDA). See, e.g., Diagnostic MolecularMicrobiology: Principles and Applications, Persing, et al., eds.,American Society for Microbiology, Washington, D.C. (1993). The productof amplification is termed an amplicon.

The term “conservatively modified variants” applies to both amino acidand nucleic acid sequences. With respect to particular nucleic acidsequences, conservatively modified variants refer to those nucleic acidsthat encode identical or conservatively modified variants of the aminoacid sequences. Because of the degeneracy of the genetic code, a largenumber of functionally identical nucleic acids encode any given protein.For instance, the codons GCA, GCC, GCG and GCU all encode the amino acidalanine. Thus, at every position where an alanine is specified by acodon, the codon can be altered to any of the corresponding codonsdescribed without altering the encoded polypeptide. Such nucleic acidvariations are “silent variations” and represent one species ofconservatively modified variation. Every nucleic acid sequence hereinthat encodes a polypeptide also describes every possible silentvariation of the nucleic acid. One of ordinary skill will recognize thateach codon in a nucleic acid (except AUG, which is ordinarily the onlycodon for methionine; one exception is Micrococcus rubens, for which GTGis the methionine codon (Ishizuka, et al., (1993) J. Gen. Microbiol.139:425-32) can be modified to yield a functionally identical molecule.Accordingly, each silent variation of a nucleic acid, which encodes apolypeptide of the present invention, is implicit in each describedpolypeptide sequence and incorporated herein by reference.

As to amino acid sequences, one of skill will recognize that individualsubstitutions, deletions or additions to a nucleic acid, peptide,polypeptide or protein sequence which alters, adds or deletes a singleamino acid or a small percentage of amino acids in the encoded sequenceis a “conservatively modified variant” when the alteration results inthe substitution of an amino acid with a chemically similar amino acid.Thus, any number of amino acid residues selected from the group ofintegers consisting of from 1 to 15 can be so altered. Thus, forexample, 1, 2, 3, 4, 5, 7 or 10 alterations can be made. Conservativelymodified variants typically provide similar biological activity as theunmodified polypeptide sequence from which they are derived. Forexample, substrate specificity, enzyme activity, or ligand/receptorbinding is generally at least 30%, 40%, 50%, 60%, 70%, 80% or 90%,preferably 60-90% of the native protein for it's native substrate.Conservative substitution tables providing functionally similar aminoacids are well known in the art.

The following six groups each contain amino acids that are conservativesubstitutions for one another:

-   -   1) Alanine (A), Serine (S), Threonine (T);    -   2) Aspartic acid (D), Glutamic acid (E);    -   3) Asparagine (N), Glutamine (Q);    -   4) Arginine (R), Lysine (K);    -   5) Isoleucine (I), Leucine (L), Methionine (M), Valine (V); and    -   6) Phenylalanine (F), Tyrosine (Y), Tryptophan (W).        See also, Creighton, Proteins, W.H. Freeman and Co. (1984).

As used herein, “consisting essentially of” means the inclusion ofadditional sequences to an object polynucleotide where the additionalsequences do not selectively hybridize, under stringent hybridizationconditions, to the same cDNA as the polynucleotide and where thehybridization conditions include a wash step in 0.1×SSC and 0.1% sodiumdodecyl sulfate at 65° C.

By “encoding” or “encoded,” with respect to a specified nucleic acid, ismeant comprising the information for translation into the specifiedprotein. A nucleic acid encoding a protein may comprise non-translatedsequences (e.g., introns) within translated regions of the nucleic acid,or may lack such intervening non-translated sequences (e.g., as incDNA). The information by which a protein is encoded is specified by theuse of codons. Typically, the amino acid sequence is encoded by thenucleic acid using the “universal” genetic code. However, variants ofthe universal code, such as is present in some plant, animal, and fungalmitochondria, the bacterium Mycoplasma capricolumn (Yamao, et al.,(1985) Proc. Natl. Acad. Sci. USA 82:2306-9) or the ciliateMacronucleus, may be used when the nucleic acid is expressed using theseorganisms.

When the nucleic acid is prepared or altered synthetically, advantagecan be taken of known codon preferences of the intended host where thenucleic acid is to be expressed. For example, although nucleic acidsequences of the present invention may be expressed in bothmonocotyledonous and dicotyledonous plant species, sequences can bemodified to account for the specific codon preferences and GC contentpreferences of monocotyledonous plants or dicotyledonous plants as thesepreferences have been shown to differ (Murray, et al., (1989) NucleicAcids Res. 17:477-98 and herein incorporated by reference). Thus, themaize preferred codon for a particular amino acid might be derived fromknown gene sequences from maize. Maize codon usage for 28 genes frommaize plants is listed in Table 4 of Murray, et al., supra.

As used herein, “heterologous” in reference to a nucleic acid is anucleic acid that originates from a foreign species, or, if from thesame species, is substantially modified from its native form incomposition and/or genomic locus by deliberate human intervention. Forexample, a promoter operably linked to a heterologous structural gene isfrom a species different from that from which the structural gene wasderived or, if from the same species, one or both are substantiallymodified from their original form. A heterologous protein may originatefrom a foreign species or, if from the same species, is substantiallymodified from its original form by deliberate human intervention.

By “host cell” is meant a cell, which comprises a heterologous nucleicacid sequence of the invention, which contains a vector and supports thereplication and/or expression of the expression vector. Host cells maybe prokaryotic cells such as E. coli, or eukaryotic cells such as yeast,insect, plant, amphibian or mammalian cells. Preferably, host cells aremonocotyledonous or dicotyledonous plant cells, including but notlimited to maize, sorghum, sunflower, soybean, wheat, alfalfa, rice,cotton, canola, barley, millet and tomato. A particularly preferredmonocotyledonous host cell is a maize host cell.

The term “hybridization complex” includes reference to a duplex nucleicacid structure formed by two single-stranded nucleic acid sequencesselectively hybridized with each other.

The term “introduced” in the context of inserting a nucleic acid into acell, means “transfection” or “transformation” or “transduction” andincludes reference to the incorporation of a nucleic acid into aeukaryotic or prokaryotic cell where the nucleic acid may beincorporated into the genome of the cell (e.g., chromosome, plasmid,plastid or mitochondrial DNA), converted into an autonomous replicon ortransiently expressed (e.g., transfected mRNA).

The terms “isolated” refers to material, such as a nucleic acid or aprotein, which is substantially or essentially free from componentswhich normally accompany or interact with it as found in its naturallyoccurring environment. The isolated material optionally comprisesmaterial not found with the material in its natural environment. Nucleicacids, which are “isolated”, as defined herein, are also referred to as“heterologous” nucleic acids. Unless otherwise stated, the term “NRnucleic acid” means a nucleic acid comprising a polynucleotide (“NRpolynucleotide”) encoding a full length or partial length NRpolypeptide.

As used herein, “nucleic acid” includes reference to adeoxyribonucleotide or ribonucleotide polymer in either single- ordouble-stranded form and unless otherwise limited, encompasses knownanalogues having the essential nature of natural nucleotides in thatthey hybridize to single-stranded nucleic acids in a manner similar tonaturally occurring nucleotides (e.g., peptide nucleic acids).

By “nucleic acid library” is meant a collection of isolated DNA or RNAmolecules, which comprise and substantially represent the entiretranscribed fraction of a genome of a specified organism. Constructionof exemplary nucleic acid libraries, such as genomic and cDNA libraries,is taught in standard molecular biology references such as Berger andKimmel, (1987) Guide To Molecular Cloning Techniques, from the seriesMethods in Enzymology, vol. 152, Academic Press, Inc., San Diego,Calif.; Sambrook, et al., (1989) Molecular Cloning: A Laboratory Manual,2^(nd) ed., vols. 1-3; and Current Protocols in Molecular Biology,Ausubel, et al., eds, Current Protocols, a joint venture between GreenePublishing Associates, Inc. and John Wiley & Sons, Inc. (1994Supplement).

As used herein “operably linked” includes reference to a functionallinkage between a first sequence, such as a promoter, and a secondsequence, wherein the promoter sequence initiates and mediatestranscription of the DNA corresponding to the second sequence.

Generally, operably linked means that the nucleic acid sequences beinglinked are contiguous and, where necessary to join two protein codingregions, contiguous and in the same reading frame.

As used herein, the term “plant” includes reference to whole plants,plant organs (e.g., leaves, stems, roots, etc.), seeds and plant cellsand progeny of same. Plant cell, as used herein includes, withoutlimitation, seeds, suspension cultures, embryos, meristematic regions,callus tissue, leaves, roots, shoots, gametophytes, sporophytes, pollenand microspores. The class of plants, which can be used in the methodsof the invention, is generally as broad as the class of higher plantsamenable to transformation techniques, including both monocotyledonousand dicotyledonous plants including species from the genera: Cucurbita,Rosa, Vitis, Juglans, Fragaria, Lotus, Medicago, Onobrychis, Trifolium,Trigonella, Vigna, Citrus, Linum, Geranium, Manihot, Daucus,Arabidopsis, Brassica, Raphanus, Sinapis, Atropa, Capsicum, Datura,Hyoscyamus, Lycopersicon, Nicotiana, Solanum, Petunia, Digitalis,Majorana, Ciahorium, Helianthus, Lactuca, Bromus, Asparagus,Antirrhinum, Heterocallis, Nemesis, Pelargonium, Panieum, Pennisetum,Ranunculus, Senecio, Salpiglossis, Cucumis, Browaalia, Glycine, Pisum,Phaseolus, Lolium, Oryza, Avena, Hordeum, Secale, Allium and Triticum. Aparticularly preferred plant is Zea mays.

As used herein, “yield” may include reference to bushels per acre of agrain crop at harvest, as adjusted for grain moisture (15% typically formaize, for example) and the volume of biomass generated (for foragecrops such as alfalfa and plant root size for multiple crops). Grainmoisture is measured in the grain at harvest. The adjusted test weightof grain is determined to be the weight in pounds per bushel, adjustedfor grain moisture level at harvest. Biomass is measured as the weightof harvestable plant material generated.

As used herein, “polynucleotide” includes reference to adeoxyribopolynucleotide, ribopolynucleotide or analogs thereof that havethe essential nature of a natural ribonucleotide in that they hybridize,under stringent hybridization conditions, to substantially the samenucleotide sequence as naturally occurring nucleotides and/or allowtranslation into the same amino acid(s) as the naturally occurringnucleotide(s). A polynucleotide can be full-length or a subsequence of anative or heterologous structural or regulatory gene. Unless otherwiseindicated, the term includes reference to the specified sequence as wellas the complementary sequence thereof. Thus, DNAs or RNAs with backbonesmodified for stability or for other reasons are “polynucleotides” asthat term is intended herein. Moreover, DNAs or RNAs comprising unusualbases, such as inosine, or modified bases, such as tritylated bases, toname just two examples, are polynucleotides as the term is used herein.It will be appreciated that a great variety of modifications have beenmade to DNA and RNA that serve many useful purposes known to those ofskill in the art. The term polynucleotide as it is employed hereinembraces such chemically, enzymatically or metabolically modified formsof polynucleotides, as well as the chemical forms of DNA and RNAcharacteristic of viruses and cells, including inter alia, simple andcomplex cells.

The terms “polypeptide,” “peptide” and “protein” are usedinterchangeably herein to refer to a polymer of amino acid residues. Theterms apply to amino acid polymers in which one or more amino acidresidue is an artificial chemical analogue of a corresponding naturallyoccurring amino acid, as well as to naturally occurring amino acidpolymers.

As used herein “promoter” includes reference to a region of DNA upstreamfrom the start of transcription and involved in recognition and bindingof RNA polymerase and other proteins to initiate transcription. A “plantpromoter” is a promoter capable of initiating transcription in plantcells. Exemplary plant promoters include, but are not limited to, thosethat are obtained from plants, plant viruses and bacteria which comprisegenes expressed in plant cells such Agrobacterium or Rhizobium. Examplesare promoters that preferentially initiate transcription in certaintissues, such as leaves, roots, seeds, fibres, xylem vessels, tracheidsor sclerenchyma. Such promoters are referred to as “tissue preferred.” A“cell type” specific promoter primarily drives expression in certaincell types in one or more organs, for example, vascular cells in rootsor leaves. An “inducible” or “regulatable” promoter is a promoter, whichis under environmental control. Examples of environmental conditionsthat may effect transcription by inducible promoters include anaerobicconditions or the presence of light. Another type of promoter is adevelopmentally regulated promoter, for example, a promoter that drivesexpression during pollen development. Tissue preferred, cell typespecific, developmentally regulated and inducible promoters constitutethe class of “non-constitutive” promoters. A “constitutive” promoter isa promoter, which is active under most environmental conditions, forexample, the ubiquitin gene promoter UBI (GenBank Accesssion NumberS94464).

As used herein, the term nitrate reductase (NR) includes but is notlimited to the sequences disclosed herein, such as NR, theirconservatively modified variants, regardless of source and any othervariants which retain the biological properties of the NR, for example,NR activity as disclosed herein. The term “NR polypeptide” refers to oneor more amino acid sequences. The term is also inclusive of fragments,variants, homologs, alleles or precursors (e.g., preproproteins orproproteins) thereof. A “NR protein” comprises a NR polypeptide. Unlessotherwise stated, the term “NR nucleic acid” means a nucleic acidcomprising a polynucleotide (“NR polynucleotide”) encoding a NRpolypeptide.

As used interchangeably herein, a “NR activity”, “biological activity ofNR” or “functional activity of NR”, refers to an activity exerted by aNR protein, polypeptide or portion thereof as determined in vivo, or invitro, according to standard techniques. In one aspect, a NR activity isthe reduction of nitrate to nitrite. In one aspect, NR activity includesbut is not limited to increased nitrate reduction rate and/orspecificity for nitrate, for example, decreased K_(m) for nitrate andNADH, increased velocity (V_(max)) for nitrate reduction and the like ascompared to NR activity of an endogenous NR of a crop plant of interest.In another aspect, NR activity includes but is not limited to increasingNUE and/or plant productivity/yield as compared to a control plant. NUEmay be inferred from amount and/or rate of nitrogen uptake from the soilor medium as described herein in Example 11.

The expression level of the NR polypeptide may be measured directly, forexample, by measuring the level of the NR polypeptide by Western in theplant, or indirectly, for example, by measuring the NR activity of theNR polypeptide in the plant. Methods for determining the NR activity maybe determined using standard techniques such as Hageman, et al., MethodsEnzymol. (1971) 23:491-503, Tucker, et al., (2004) Planta 219:277-285and Scheible, et al., (1997) Plant J. 11:671-691, including theevaluation of activity in various expression systems, for example ofXenopus oocytes (see, Miller, et al., (2000) Biochimica et BiophysicaActa 1465:343-358.) or yeast such as Pichia pastoris (U.S. ProvisionalPatent Application Ser. No. 60/944,223 filed Jun. 15, 2007), NR activitymay also include evaluation of phenotypic changes, such as increased ormaintained yield or NUE in a plant grown under nitrate limitingconditions such as lower nitrogen fertility. Examples of phenoypicchanges include but are not limited to increased ear size in maize,increased ear growth rate, increased biomass, higher grain yields,synchronous flowering so that pollen is shed at approximately the sametime as silking, enhanced root growth, increased seed size, increasedseed weight, seed with increased embryo size, increased leaf size,increased seedling vigor, enhanced silk emergence and greaterchlorophyll content (greener).

Maintained or increased yield may be achieved through NRs of the presentinvention. Thus, modulation of NR activity of the NRs of the presentinvention in a plant cell provides a novel strategy for maintaining orincreasing yield or NUE of a plant grown under limited nitrogen supplyor lower nitrogen fertility

Accordingly, the present invention further provides plants havingincreased yield or a maintained yield when grown under limited nitrogenfertility. In some embodiments, the plants having an increased ormaintained yield when grown under limited nitrogen fertility have amodulated level/activity of a NR polypeptide of the invention.

A “subject plant or plant cell” is one in which genetic alteration, suchas transformation, has been effected as to a gene of interest or is aplant or plant cell which is descended from a plant or cell so alteredand which comprises the alteration. A “control” or “control plant” or“control plant cell” provides a reference point for measuring changes inphenotype of the subject plant or plant cell.

A control plant or plant cell may comprise, for example: (a) a wild-typeplant or cell, i.e., of the same genotype as the starting material forthe genetic alteration which resulted in the subject plant or cell; (b)a plant or plant cell of the same genotype as the starting material butwhich has been transformed with a null construct (i.e., with a constructwhich has no known effect on the trait of interest, such as a constructcomprising a marker gene); (c) a plant or plant cell which is anon-transformed segregant among progeny of a subject plant or plantcell; (d) a plant or plant cell genetically identical to the subjectplant or plant cell but which is not exposed to conditions or stimulithat would induce expression of the gene of interest or (e) the subjectplant or plant cell itself, under conditions in which the gene ofinterest is not expressed.

As used herein “recombinant” includes reference to a cell or vector,that has been modified by the introduction of a heterologous nucleicacid or that the cell is derived from a cell so modified. Thus, forexample, recombinant cells express genes that are not found in identicalform within the native (non-recombinant) form of the cell or expressnative genes that are otherwise abnormally expressed, under expressed ornot expressed at all as a result of deliberate human intervention; ormay have reduced or eliminated expression of a native gene. The term“recombinant” as used herein does not encompass the alteration of thecell or vector by naturally occurring events (e.g., spontaneousmutation, natural transformation/transduction/transposition) such asthose occurring without deliberate human intervention.

As used herein, a “recombinant expression cassette” is a nucleic acidconstruct, generated recombinantly or synthetically, with a series ofspecified nucleic acid elements, which permit transcription of aparticular nucleic acid in a target cell. The recombinant expressioncassette can be incorporated into a plasmid, chromosome, mitochondrialDNA, plastid DNA, virus or nucleic acid fragment. Typically, therecombinant expression cassette portion of an expression vectorincludes, among other sequences, a nucleic acid to be transcribed and apromoter.

The terms “residue” or “amino acid residue” or “amino acid” are usedinterchangeably herein to refer to an amino acid that is incorporatedinto a protein, polypeptide or peptide (collectively “protein”). Theamino acid may be a naturally occurring amino acid and, unless otherwiselimited, may encompass known analogs of natural amino acids that canfunction in a similar manner as naturally occurring amino acids.

The term “selectively hybridizes” includes reference to hybridization,under stringent hybridization conditions, of a nucleic acid sequence toa specified nucleic acid target sequence to a detectably greater degree(e.g., at least 2-fold over background) than its hybridization tonon-target nucleic acid sequences and to the substantial exclusion ofnon-target nucleic acids. Selectively hybridizing sequences typicallyhave about at least 40% sequence identity, preferably 60-90% sequenceidentity and most preferably 100% sequence identity (i.e.,complementary) with each other.

The terms “stringent conditions” or “stringent hybridization conditions”include reference to conditions under which a probe will hybridize toits target sequence, to a detectably greater degree than other sequences(e.g., at least 2-fold over background). Stringent conditions aresequence-dependent and will be different in different circumstances. Bycontrolling the stringency of the hybridization and/or washingconditions, target sequences can be identified which can be up to 100%complementary to the probe (homologous probing). Alternatively,stringency conditions can be adjusted to allow some mismatching insequences so that lower degrees of similarity are detected (heterologousprobing). Optimally, the probe is approximately 500 nucleotides inlength, but can vary greatly in length from less than 500 nucleotides toequal to the entire length of the target sequence.

Typically, stringent conditions will be those in which the saltconcentration is less than about 1.5 M Na ion, typically about 0.01 to1.0 M Na ion concentration (or other salts) at pH 7.0 to 8.3 and thetemperature is at least about 30° C. for short probes (e.g., 10 to 50nucleotides) and at least about 60° C. for long probes (e.g., greaterthan 50 nucleotides). Stringent conditions may also be achieved with theaddition of destabilizing agents such as formamide or Denhardt's.Exemplary low stringency conditions include hybridization with a buffersolution of 30 to 35% formamide, 1 M NaCl, 1% SDS (sodium dodecylsulphate) at 37° C. and a wash in 1× to 2×SSC (20×SSC=3.0 M NaCl/0.3 Mtrisodium citrate) at 50 to 55° C. Exemplary moderate stringencyconditions include hybridization in 40 to 45% formamide, 1 M NaCl, 1%SDS at 37° C. and a wash in 0.5× to 1×SSC at 55 to 60° C. Exemplary highstringency conditions include hybridization in 50% formamide, 1 M NaCl,1% SDS at 37° C. and a wash in 0.1×SSC at 60 to 65° C. Specificity istypically the function of post-hybridization washes, the criticalfactors being the ionic strength and temperature of the final washsolution. For DNA-DNA hybrids, the T_(m) can be approximated from theequation of Meinkoth and Wahl, (1984) Anal. Biochem. 138:267-84:T_(m)=81.5° C.+16.6 (log M)+0.41 (% GC)−0.61 (% form)−500/L; where M isthe molarity of monovalent cations, % GC is the percentage of guanosineand cytosine nucleotides in the DNA, % form is the percentage offormamide in the hybridization solution, and L is the length of thehybrid in base pairs. The T_(m) is the temperature (under defined ionicstrength and pH) at which 50% of a complementary target sequencehybridizes to a perfectly matched probe. T_(m) is reduced by about 1° C.for each 1% of mismatching; thus, T_(m), hybridization and/or washconditions can be adjusted to hybridize to sequences of the desiredidentity. For example, if sequences with ≧90% identity are sought, theT_(m) can be decreased 10° C. Generally, stringent conditions areselected to be about 5° C. lower than the thermal melting point (T_(m))for the specific sequence and its complement at a defined ionic strengthand pH. However, severely stringent conditions can utilize ahybridization and/or wash at 1, 2, 3 or 4° C. lower than the thermalmelting point (T_(m)); moderately stringent conditions can utilize ahybridization and/or wash at 6, 7, 8, 9 or 10° C. lower than the thermalmelting point (T_(m)); low stringency conditions can utilize ahybridization and/or wash at 11, 12, 13, 14, 15 or 20° C. lower than thethermal melting point (T_(m)). Using the equation, hybridization andwash compositions, and desired T_(m), those of ordinary skill willunderstand that variations in the stringency of hybridization and/orwash solutions are inherently described. If the desired degree ofmismatching results in a T_(m) of less than 45° C. (aqueous solution) or32° C. (formamide solution) it is preferred to increase the SSCconcentration so that a higher temperature can be used. An extensiveguide to the hybridization of nucleic acids is found in Tijssen,Laboratory Techniques in Biochemistry and MolecularBiology—Hybridization with Nucleic Acid Probes, part I, chapter 2,“Overview of principles of hybridization and the strategy of nucleicacid probe assays,” Elsevier, New York (1993); and Current Protocols inMolecular Biology, chapter 2, Ausubel, et al., eds, Greene Publishingand Wiley-Interscience, New York (1995). Unless otherwise stated, in thepresent application high stringency is defined as hybridization in4×SSC, 5×Denhardt's (5 g Ficoll, 5 g polyvinypyrrolidone, 5 g bovineserum albumin in 500 ml of water), 0.1 mg/ml boiled salmon sperm DNA and25 mM Na phosphate at 65° C. and a wash in 0.1×SSC, 0.1% SDS at 65° C.

As used herein, “transgenic plant” includes reference to a plant, whichcomprises within its genome a heterologous polynucleotide. Generally,the heterologous polynucleotide is stably integrated within the genomesuch that the polynucleotide is passed on to successive generations. Theheterologous polynucleotide may be integrated into the genome alone oras part of a recombinant expression cassette. “Transgenic” is usedherein to include any cell, cell line, callus, tissue, plant part orplant, the genotype of which has been altered by the presence ofheterologous nucleic acid including those transgenics initially soaltered as well as those created by sexual crosses or asexualpropagation from the initial transgenic. The term “transgenic” as usedherein does not encompass the alteration of the genome (chromosomal orextra-chromosomal) by conventional plant breeding methods or bynaturally occurring events such as random cross-fertilization,non-recombinant viral infection, non-recombinant bacterialtransformation, non-recombinant transposition or spontaneous mutation.

As used herein, “vector” includes reference to a nucleic acid used intransfection of a host cell and into which can be inserted apolynucleotide. Vectors are often replicons. Expression vectors permittranscription of a nucleic acid inserted therein.

The following terms are used to describe the sequence relationshipsbetween two or more nucleic acids or polynucleotides or polypeptides:(a) “reference sequence,” (b) “comparison window,” (c) “sequenceidentity,” (d) “percentage of sequence identity” and (e) “substantialidentity.”

As used herein, “reference sequence” is a defined sequence used as abasis for sequence comparison. A reference sequence may be a subset orthe entirety of a specified sequence; for example, as a segment of afull-length cDNA or gene sequence or the complete cDNA or gene sequence.

As used herein, “comparison window” means includes reference to acontiguous and specified segment of a polynucleotide sequence, whereinthe polynucleotide sequence may be compared to a reference sequence andwherein the portion of the polynucleotide sequence in the comparisonwindow may comprise additions or deletions (i.e., gaps) compared to thereference sequence (which does not comprise additions or deletions) foroptimal alignment of the two sequences. Generally, the comparison windowis at least 20 contiguous nucleotides in length, and optionally can be30, 40, 50, 100 or longer. Those of skill in the art understand that toavoid a high similarity to a reference sequence due to inclusion of gapsin the polynucleotide sequence a gap penalty is typically introduced andis subtracted from the number of matches.

Methods of alignment of nucleotide and amino acid sequences forcomparison are well known in the art. The local homology algorithm(BESTFIT) of Smith and Waterman, (1981) Adv. Appl. Math 2:482, mayconduct optimal alignment of sequences for comparison; by the homologyalignment algorithm (GAP) of Needleman and Wunsch, (1970) J. Mol. Biol.48:443-53; by the search for similarity method (Tfasta and Fasta) ofPearson and Lipman, (1988) Proc. Natl. Acad. Sci. USA 85:2444; bycomputerized implementations of these algorithms, including, but notlimited to: CLUSTAL in the PC/Gene program by Intelligenetics, MountainView, Calif., GAP, BESTFIT, BLAST, FASTA and TFASTA in the WisconsinGenetics Software Package®, Version 8 (available from Genetics ComputerGroup (GCG® programs (Accelrys, Inc., San Diego, Calif.).). The CLUSTALprogram is well described by Higgins and Sharp, (1988) Gene 73:237-44;Higgins and Sharp, (1989) CABIOS 5:151-3; Corpet, et al., (1988) NucleicAcids Res. 16:10881-90; Huang, et al., (1992) Computer Applications inthe Biosciences 8:155-65 and Pearson, et al., (1994) Meth. Mol. Biol.24:307-31. The preferred program to use for optimal global alignment ofmultiple sequences is PileUp (Feng and Doolittle, (1987) J. Mol. Evol.,25:351-60 which is similar to the method described by Higgins and Sharp,(1989) CABIOS 5:151-53 and hereby incorporated by reference). The BLASTfamily of programs which can be used for database similarity searchesincludes: BLASTN for nucleotide query sequences against nucleotidedatabase sequences; BLASTX for nucleotide query sequences againstprotein database sequences; BLASTP for protein query sequences againstprotein database sequences; TBLASTN for protein query sequences againstnucleotide database sequences and TBLASTX for nucleotide query sequencesagainst nucleotide database sequences. See, Current Protocols inMolecular Biology, Chapter 19, Ausubel et al., eds., Greene Publishingand Wiley-Interscience, New York (1995).

GAP uses the algorithm of Needleman and Wunsch, supra, to find thealignment of two complete sequences that maximizes the number of matchesand minimizes the number of gaps. GAP considers all possible alignmentsand gap positions and creates the alignment with the largest number ofmatched bases and the fewest gaps. It allows for the provision of a gapcreation penalty and a gap extension penalty in units of matched bases.GAP must make a profit of gap creation penalty number of matches foreach gap it inserts. If a gap extension penalty greater than zero ischosen, GAP must, in addition, make a profit for each gap inserted ofthe length of the gap times the gap extension penalty. Default gapcreation penalty values and gap extension penalty values in Version 10of the Wisconsin Genetics Software Package® are 8 and 2, respectively.The gap creation and gap extension penalties can be expressed as aninteger selected from the group of integers consisting of from 0 to 100.Thus, for example, the gap creation and gap extension penalties can be0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, 40, 50 or greater.

GAP presents one member of the family of best alignments. There may bemany members of this family, but no other member has a better quality.GAP displays four figures of merit for alignments: Quality, Ratio,Identity and Similarity. The Quality is the metric maximized in order toalign the sequences. Ratio is the quality divided by the number of basesin the shorter segment. Percent Identity is the percent of the symbolsthat actually match. Percent Similarity is the percent of the symbolsthat are similar. Symbols that are across from gaps are ignored. Asimilarity is scored when the scoring matrix value for a pair of symbolsis greater than or equal to 0.50, the similarity threshold. The scoringmatrix used in Version 10 of the Wisconsin Genetics Software Package® isBLOSUM62 (see, Henikoff and Henikoff, (1989) Proc. Natl. Acad. Sci. USA89:10915).

Unless otherwise stated, sequence identity/similarity values providedherein refer to the value obtained using the BLAST 2.0 suite of programsusing default parameters (Altschul, et al., (1997) Nucleic Acids Res.25:3389-402).

As those of ordinary skill in the art will understand, BLAST searchesassume that proteins can be modeled as random sequences. However, manyreal proteins comprise regions of nonrandom sequences, which may behomopolymeric tracts, short-period repeats or regions enriched in one ormore amino acids. Such low-complexity regions may be aligned betweenunrelated proteins even though other regions of the protein are entirelydissimilar. A number of low-complexity filter programs can be employedto reduce such low-complexity alignments. For example, the SEG (Wootenand Federhen, (1993) Comput. Chem. 17:149-63) and XNU (Claverie andStates, (1993) Comput. Chem. 17:191-201) low-complexity filters can beemployed alone or in combination.

As used herein, “sequence identity” or “identity” in the context of twonucleic acid or polypeptide sequences includes reference to the residuesin the two sequences, which are the same when aligned for maximumcorrespondence over a specified comparison window. When percentage ofsequence identity is used in reference to proteins it is recognized thatresidue positions which are not identical often differ by conservativeamino acid substitutions, where amino acid residues are substituted forother amino acid residues with similar chemical properties (e.g., chargeor hydrophobicity) and therefore do not change the functional propertiesof the molecule. Where sequences differ in conservative substitutions,the percent sequence identity may be adjusted upwards to correct for theconservative nature of the substitution. Sequences, which differ by suchconservative substitutions, are said to have “sequence similarity” or“similarity.” Means for making this adjustment are well known to thoseof skill in the art. Typically this involves scoring a conservativesubstitution as a partial rather than a full mismatch, therebyincreasing the percentage sequence identity. Thus, for example, where anidentical amino acid is given a score of 1 and a non-conservativesubstitution is given a score of zero, a conservative substitution isgiven a score between zero and 1. The scoring of conservativesubstitutions is calculated, e.g., according to the algorithm of Meyersand Miller, (1988) Computer Applic. Biol. Sci. 4:11-17, e.g., asimplemented in the program PC/GENE (Intelligenetics, Mountain View,Calif., USA).

As used herein, “percentage of sequence identity” means the valuedetermined by comparing two optimally aligned sequences over acomparison window, wherein the portion of the polynucleotide sequence inthe comparison window may comprise additions or deletions (i.e., gaps)as compared to the reference sequence (which does not comprise additionsor deletions) for optimal alignment of the two sequences. The percentageis calculated by determining the number of positions at which theidentical nucleic acid base or amino acid residue occurs in bothsequences to yield the number of matched positions, dividing the numberof matched positions by the total number of positions in the window ofcomparison and multiplying the result by 100 to yield the percentage ofsequence identity.

The term “substantial identity” of polynucleotide sequences means that apolynucleotide comprises a sequence that has between 50-100% sequenceidentity, preferably at least 50% sequence identity, preferably at least60% sequence identity, preferably at least 70%, more preferably at least80%, more preferably at least 90% and most preferably at least 95%,compared to a reference sequence using one of the alignment programsdescribed using standard parameters. One of skill will recognize thatthese values can be appropriately adjusted to determine correspondingidentity of proteins encoded by two nucleotide sequences by taking intoaccount codon degeneracy, amino acid similarity, reading framepositioning and the like. Substantial identity of amino acid sequencesfor these purposes normally means sequence identity of between 55-100%,preferably at least 55%, preferably at least 60%, more preferably atleast 70%, 80%, 90% and most preferably at least 95%.

Another indication that nucleotide sequences are substantially identicalis if two molecules hybridize to each other under stringent conditions.The degeneracy of the genetic code allows for many amino acidssubstitutions that lead to variety in the nucleotide sequence that codefor the same amino acid, hence it is possible that the DNA sequencecould code for the same polypeptide but not hybridize to each otherunder stringent conditions. This may occur, e.g., when a copy of anucleic acid is created using the maximum codon degeneracy permitted bythe genetic code. One indication that two nucleic acid sequences aresubstantially identical is that the polypeptide, which the first nucleicacid encodes, is immunologically cross reactive with the polypeptideencoded by the second nucleic acid.

The terms “substantial identity” in the context of a peptide indicatesthat a peptide comprises a sequence with between 55-100% sequenceidentity to a reference sequence preferably at least 55% sequenceidentity, preferably 60% preferably 70%, more preferably 80%, mostpreferably at least 90% or 95% sequence identity to the referencesequence over a specified comparison window. Preferably, optimalalignment is conducted using the homology alignment algorithm ofNeedleman and Wunsch, supra. An indication that two peptide sequencesare substantially identical is that one peptide is immunologicallyreactive with antibodies raised against the second peptide. Thus, apeptide is substantially identical to a second peptide, for example,where the two peptides differ only by a conservative substitution. Inaddition, a peptide can be substantially identical to a second peptidewhen they differ by a non-conservative change if the epitope that theantibody recognizes is substantially identical. Peptides, which are“substantially similar” share sequences as, noted above except thatresidue positions, which are not identical, may differ by conservativeamino acid changes.

Nucleic Acids

The present invention provides, inter alia, isolated nucleic acids ofRNA, DNA and analogs and/or chimeras thereof, comprising a NRpolynucleotide.

The present invention also includes polynucleotides optimized forexpression in different organisms. For example, for expression of thepolynucleotide in a maize plant, the sequence can be altered to accountfor specific codon preferences and to alter GC content as according toMurray, et al, supra. Maize codon usage for 28 genes from maize plantsis listed in Table 4 of Murray, et al., supra.

The NR nucleic acids of the present invention comprise isolated NRpolynucleotides which are inclusive of:

-   -   (a) a polynucleotide encoding a NR polypeptide and        conservatively modified and polymorphic variants thereof;    -   (b) a polynucleotide having at least 70% sequence identity with        polynucleotides of (a);    -   (c) complementary sequences of polynucleotides of (a) or (b).

Construction of Nucleic Acids

The isolated nucleic acids of the present invention can be made using(a) standard recombinant methods, (b) synthetic techniques orcombinations thereof. In some embodiments, the polynucleotides of thepresent invention will be cloned, amplified or otherwise constructedfrom a fungus or bacteria.

The nucleic acids may conveniently comprise sequences in addition to apolynucleotide of the present invention. For example, a multi-cloningsite comprising one or more endonuclease restriction sites may beinserted into the nucleic acid to aid in isolation of thepolynucleotide. Also, translatable sequences may be inserted to aid inthe isolation of the translated polynucleotide of the present invention.For example, a hexa-histidine marker sequence provides a convenientmeans to purify the proteins of the present invention. The nucleic acidof the present invention—excluding the polynucleotide sequence—isoptionally a vector, adapter or linker for cloning and/or expression ofa polynucleotide of the present invention. Additional sequences may beadded to such cloning and/or expression sequences to optimize theirfunction in cloning and/or expression, to aid in isolation of thepolynucleotide, or to improve the introduction of the polynucleotideinto a cell. Typically, the length of a nucleic acid of the presentinvention less the length of its polynucleotide of the present inventionis less than 20 kilobase pairs, often less than 15 kb and frequentlyless than 10 kb. Use of cloning vectors, expression vectors, adaptersand linkers is well known in the art. Exemplary nucleic acids includesuch vectors as: M13, lambda ZAP Express, lambda ZAP II, lambda gt10,lambda gt11, pBK-CMV, pBK-RSV, pBluescript II, lambda DASH II, lambdaEMBL 3, lambda EMBL 4, pWE15, SuperCos 1, SurfZap, Uni-ZAP, pBC, pBS+/−,pSG5, pBK, pCR-Script, pET, pSPUTK, p3′SS, pGEM, pSK+/−, pGEX, pSPORTIand II, pOPRSVI CAT, pOPI3 CAT, pXT1, pSG5, pPbac, pMbac, pMC1neo,pOG44, pOG45, pFRTβGAL, pNEOβGAL, pRS403, pRS404, pRS405, pRS406,pRS413, pRS414, pRS415, pRS416, lambda MOSSIox and lambda MOSEIox.Optional vectors for the present invention, include but are not limitedto, lambda ZAP II and pGEX. For a description of various nucleic acidssee, e.g., Stratagene Cloning Systems, Catalogs 1995, 1996, 1997 (LaJolla, Calif.) and Amersham Life Sciences, Inc, Catalog '97 (ArlingtonHeights, Ill.).

Synthetic Methods for Constructing Nucleic Acids

The isolated nucleic acids of the present invention can also be preparedby direct chemical synthesis by methods such as the phosphotriestermethod of Narang, et al., (1979) Meth. Enzymol. 68:90-9; thephosphodiester method of Brown, et al., (1979) Meth. Enzymol. 68:109-51;the diethylphosphoramidite method of Beaucage, et al., (1981) Tetra.Letts. 22(20):1859-62; the solid phase phosphoramidite triester methoddescribed by Beaucage, et al., supra, e.g., using an automatedsynthesizer, e.g., as described in Needham-VanDevanter, et al., (1984)Nucleic Acids Res. 12:6159-68 and the solid support method of U.S. Pat.No. 4,458,066. Chemical synthesis generally produces a single strandedoligonucleotide. This may be converted into double stranded DNA byhybridization with a complementary sequence or by polymerization with aDNA polymerase using the single strand as a template. One of skill willrecognize that while chemical synthesis of DNA is limited to sequencesof about 100 bases, longer sequences may be obtained by the ligation ofshorter sequences.

UTRs and Codon Preference

In general, translational efficiency has been found to be regulated byspecific sequence elements in the 5′ non-coding or untranslated region(5′ UTR) of the RNA. Positive sequence motifs include translationalinitiation consensus sequences (Kozak, (1987) Nucleic Acids Res.15:8125) and the 5<G> 7 methyl GpppG RNA cap structure (Drummond, etal., (1985) Nucleic Acids Res. 13:7375). Negative elements includestable intramolecular 5′ UTR stem-loop structures (Muesing, et al.,(1987) Cell 48:691) and AUG sequences or short open reading framespreceded by an appropriate AUG in the 5′ UTR (Kozak, supra, Rao, et al.,(1988) Mol. and Cell. Biol. 8:284). Accordingly, the present inventionprovides 5′ and/or 3′ UTR regions for modulation of translation ofheterologous coding sequences.

Further, the polypeptide-encoding segments of the polynucleotides of thepresent invention can be modified to alter codon usage. Altered codonusage can be employed to alter translational efficiency and/or tooptimize the coding sequence for expression in a desired host or tooptimize the codon usage in a heterologous sequence for expression inmaize. Codon usage in the coding regions of the polynucleotides of thepresent invention can be analyzed statistically using commerciallyavailable software packages such as “Codon Preference” available fromthe University of Wisconsin Genetics Computer Group. See, Devereaux, etal., (1984) Nucleic Acids Res. 12:387-395) or MacVector 4.1 (EastmanKodak Co., New Haven, Conn.). Thus, the present invention provides acodon usage frequency characteristic of the coding region of at leastone of the polynucleotides of the present invention. The number ofpolynucleotides (3 nucleotides per amino acid) that can be used todetermine a codon usage frequency can be any integer from 3 to thenumber of polynucleotides of the present invention as provided herein.Optionally, the polynucleotides will be full-length sequences. Anexemplary number of sequences for statistical analysis can be at least1, 5, 10, 20, 50 or 100.

Sequence Shuffling

The present invention provides methods for sequence shuffling usingpolynucleotides of the present invention and compositions resultingtherefrom. Sequence shuffling is described in PCT Publication Number96/19256. See also, Zhang, et al., (1997) Proc. Natl. Acad. Sci. USA94:4504-9 and Zhao, et al., (1998) Nature Biotech 16:258-61. Generally,sequence shuffling provides a means for generating libraries ofpolynucleotides having a desired characteristic, which can be selectedor screened for. Libraries of recombinant polynucleotides are generatedfrom a population of related sequence polynucleotides, which comprisesequence regions, which have substantial sequence identity and can behomologously recombined in vitro or in vivo. The population ofsequence-recombined polynucleotides comprises a subpopulation ofpolynucleotides which possess desired or advantageous characteristicsand which can be selected by a suitable selection or screening method.The characteristics can be any property or attribute capable of beingselected for or detected in a screening system and may includeproperties of: an encoded protein, a transcriptional element, a sequencecontrolling transcription, RNA processing, RNA stability, chromatinconformation, translation or other expression property of a gene ortransgene, a replicative element, a protein-binding element, or thelike, such as any feature which confers a selectable or detectableproperty. In some embodiments, the selected characteristic will be analtered K_(m) and/or K_(cat) over the wild-type protein as providedherein. In other embodiments, a protein or polynucleotide generated fromsequence shuffling will have a ligand binding affinity greater than thenon-shuffled wild-type polynucleotide. In yet other embodiments, aprotein or polynucleotide generated from sequence shuffling will have analtered pH optimum as compared to the non-shuffled wild-typepolynucleotide. The increase in such properties can be at least 110%,120%, 130%, 140% or greater than 150% of the wild-type value.

Recombinant Expression Cassettes

The present invention further provides recombinant expression cassettescomprising a nucleic acid of the present invention. A nucleic acidsequence coding for the desired polynucleotide of the present invention,for example a cDNA or a genomic sequence encoding a polypeptide longenough to code for an active protein of the present invention, can beused to construct a recombinant expression cassette which can beintroduced into the desired host cell. A recombinant expression cassettewill typically comprise a polynucleotide of the present inventionoperably linked to transcriptional initiation regulatory sequences whichwill direct the transcription of the polynucleotide in the intended hostcell, such as tissues of a transformed plant.

For example, plant expression vectors may include (1) a cloned plantgene under the transcriptional control of 5′ and 3′ regulatory sequencesand (2) a dominant selectable marker. Such plant expression vectors mayalso contain, if desired, a promoter regulatory region (e.g., oneconferring inducible or constitutive, environmentally- ordevelopmentally-regulated, or cell- or tissue-specific/selectiveexpression), a transcription initiation start site, a ribosome bindingsite, an RNA processing signal, a transcription termination site and/ora polyadenylation signal.

A number of promoters can be used in the practice of the invention,including the native promoter of the endogenous NR polynucleotidesequence of the crop plant of interest. The promoters can be selectedbased on the desired outcome. The nucleic acids can be combined withconstitutive, tissue-preferred, inducible, or other promoters forexpression in plants.

A plant promoter or promoter fragment can be employed which will directexpression of a polynucleotide of the present invention in all tissuesof a regenerated plant. Such promoters are referred to herein as“constitutive” promoters and are active under most environmentalconditions and states of development or cell differentiation. Examplesof constitutive promoters include the 1′- or 2′-promoter derived fromT-DNA of Agrobacterium tumefaciens, the Smas promoter, the cinnamylalcohol dehydrogenase promoter (U.S. Pat. No. 5,683,439), the Nospromoter, the rubisco promoter, the GRP1-8 promoter, the 35S promoterfrom cauliflower mosaic virus (CaMV), as described in Odell, et al.,(1985) Nature 313:810-2; rice actin (McElroy, et al., (1990) Plant Cell163-171); ubiquitin (Christensen, et al., (1992) Plant Mol. Biol.12:619-632 and Christensen, et al., (1992) Plant Mol. Biol. 18:675-89);pEMU (Last, et al., (1991) Theor. Appl. Genet. 81:581-8); MAS (Velten,et al., (1984) EMBO J. 3:2723-30) and maize H3 histone (Lepetit, et al.,(1992) Mol. Gen. Genet. 231:276-85 and Atanassvoa, et al., (1992) PlantJournal 2(3):291-300); ALS promoter, as described in PCT ApplicationNumber WO 1996/30530 and other transcription initiation regions fromvarious plant genes known to those of skill. For the present inventionubiquitin is the preferred promoter for expression in monocot plants.

Tissue-preferred promoters can be utilized to target enhanced type A RRexpression within a particular plant tissue. By “tissue-preferred” isintended to mean that expression is predominately in a particulartissue, albeit not necessarily exclusively in that tissue.Tissue-preferred promoters include Yamamoto, et al., (1997) Plant J.12(2):255-265; Kawamata, et al., (1997) Plant Cell Physiol.38(7):792-803; Hansen, et al., (1997) Mol. Gen Genet. 255(3):337-353;Russell, et al., (1997) Transgenic Res. 6(2):157-168; Rinehart, et al.,(1996) Plant Physiol. 112(3):1331-1351; Van Camp, et al., (1996) PlantPhysiol. 112(2):525-535; Canevascini, et al., (1996) Plant Physiol.112(2):513-525; Yamamoto, et al., (1995) Plant Cell Physiol.35(5):773-778; Lam, (1995) Results Probl. Cell Differ. 20:181-196;Orozco, et al., (1993) Plant Mol. Biol. 23(6):1129-1138; Matsuoka, etal., (1993) Proc Natl. Acad. Sci. USA 90(20):9586-9590 andGuevara-Garcia, et al., (1993) Plant J. 5(3):595-505. Such promoters canbe modified, if necessary, for weak expression. See, also, US PatentApplication Number 2003/0074698, herein incorporated by reference.

A mesophyllic cell preferred promoter includes but is not limited topromoters such as known phosphoenopyruvate decarboxylase (PEPC)promoters or putative PEPC promoters from any number of species, forexample, Zea mays, Oryza sativa, Arabidopsis thaliana, Glycine max orSorghum bicolor. Examples include Zea mays PEPC of GenBank AccessionNumber gi:116268332_HTG AC190686, (SEQ ID NO: 25) and gCAT GSS compositesequence (SEQ ID NO: 30); Oryza sativa PEPC of GenBank Accession Numbergi|20804452|dbj|AP003052.31 (SEQ ID NO: 26); Arabidopsis thaliana PEPCof GenBank Accession Number gi|55416531 dbj|AP000370.1|AP000370 (SEQ IDNO: 27); gi:7769847 (SEQ ID NO: 28) or gi|201980701gb|AC007087.7 (SEQ IDNO: 29); Glycine max (GSS contigs) (SEQ ID NOS: 31-32) or Sorghumbicolor (JGI assembly scaffold_(—)832, 89230 bp., JGI assemblyscaffold_(—)1632, SEQ ID NOS: 33-34) (1997) Plant J. 12(2):255-265;Kwon, et al., (1995) Plant Physiol. 105:357-67; Yamamoto, et al., (1995)Plant Cell Physiol. 35(5):773-778; Gotor, et al., (1993) Plant J.3:509-18; Orozco, et al., (1993) Plant Mol. Biol. 23(6):1129-1138;Baszczynski, et al., (1988) Nucl. Acid Res. 16:5732; Mitra, et al.,(1995) Plant Molecular Biology 26:35-93; Kayaya, et al., (1995)Molecular and General Genetics 258:668-675 and Matsuoka, et al., (1993)Proc. Natl. Acad. Sci. USA 90(20):9586-9590. Senescence regulatedpromoters are also of use, such as, SAM22 (Crowell, et al., (1992) PlantMol. Biol. 18:559-566). See also, U.S. Pat. No. 5,589,052, hereinincorporated by reference.

Shoot-preferred promoters include, shoot meristem-preferred promoterssuch as promoters disclosed in Weigal, et al., (1992) Cell 69:853-859;Accession Number AJ131822; Accession Number Z71981; Accession NumberAF059870, the ZAP promoter (U.S. patent application Ser. No.10/387,937), the maize tb1 promoter (Wang, et al., (1999) Nature398:236-239 and shoot-preferred promoters disclosed in McAvoy, et al.,(2003) Acta Hort. (ISHS) 625:379-385.

Root-preferred promoters are known and can be selected from the manyavailable from the literature or isolated de novo from variouscompatible species. See, for example, Hire, et al., (1992) Plant Mol.Biol. 20(2):207-218 (soybean root-specific glutamine synthetase gene);Keller and Baumgartner, (1991) Plant Cell 3(10):1051-1061 (root-specificcontrol element in the GRP 1.8 gene of French bean); Sanger, et al.,(1990) Plant Mol. Biol. 15(3):533-553 (root-specific promoter of themannopine synthase (MAS) gene of Agrobacterium tumefaciens) and Miao, etal., (1991) Plant Cell 3(1):11-22 (full-length cDNA clone encodingcytosolic glutamine synthetase (GS), which is expressed in roots androot nodules of soybean). See also, Bogusz, et al., (1990) Plant Cell2(7):633-651, where two root-specific promoters isolated from hemoglobingenes from the nitrogen-fixing nonlegume Parasponia andersonii and therelated non-nitrogen-fixing nonlegume Trema tomentosa are described. Thepromoters of these genes were linked to a β-glucuronidase reporter geneand introduced into both the nonlegume Nicotiana tabacum and the legumeLotus corniculatus and in both instances root-specific promoter activitywas preserved. Leach and Aoyagi, (1991) describe their analysis of thepromoters of the highly expressed roIC and rolD root-inducing genes ofAgrobacterium rhizogenes (see, Plant Science (Limerick) 79(1):69-76).They concluded that enhancer and tissue-preferred DNA determinants aredissociated in those promoters. Teen, et al., (1989) used gene fusion tolacZ to show that the Agrobacterium T-DNA gene encoding octopinesynthase is especially active in the epidermis of the root tip and thatthe TR2′ gene is root specific in the intact plant and stimulated bywounding in leaf tissue, an especially desirable combination ofcharacteristics for use with an insecticidal or larvicidal gene (see,EMBO J. 8(2):353-350). The TR1′ gene, fused to nptII (neomycinphosphotransferase II) showed similar characteristics. Additionalroot-preferred promoters include the VfENOD-GRP3 gene promoter (Kuster,et al., (1995) Plant Mol. Biol. 29(5):759-772); rolB promoter (Capana,et al., (1995) Plant Mol. Biol. 25(5):681-691 and the CRWAQ81root-preferred promoter with the ADH first intron (U.S. ProvisionalApplication No. 60/509,878, filed Oct. 9, 2003, herein incorporated byreference). See also, U.S. Pat. Nos. 5,837,876; 5,750,386; 5,633,363;5,559,252; 5,501,836; 5,110,732 and 5,023,179.

Alternatively, the plant promoter can direct expression of apolynucleotide of the present invention in a specific tissue or may beotherwise under more precise environmental or developmental control.Such promoters are referred to here as “inducible” promoters.Environmental conditions that may effect transcription by induciblepromoters include pathogen attack, anaerobic conditions or the presenceof light. Examples of inducible promoters are the Adh1 promoter, whichis inducible by hypoxia or cold stress, the Hsp70 promoter, which isinducible by heat stress, and the PPDK promoter, which is inducible bylight.

Examples of promoters under developmental control include promoters thatinitiate transcription only, or preferentially, in certain tissues, suchas leaves, roots, fruit, seeds or flowers. The operation of a promotermay also vary depending on its location in the genome. Thus, aninducible promoter may become fully or partially constitutive in certainlocations.

If polypeptide expression is desired, it is generally desirable toinclude a polyadenylation region at the 3′-end of a polynucleotidecoding region. The polyadenylation region can be derived from a varietyof plant genes or from T-DNA. The 3′ end sequence to be added can bederived from, for example, the nopaline synthase or octopine synthasegenes or alternatively from another plant gene or less preferably fromany other eukaryotic gene. Examples of such regulatory elements include,but are not limited to, 3′ termination and/or polyadenylation regionssuch as those of the Agrobacterium tumefaciens nopaline synthase (nos)gene (Bevan, et al., (1983) Nucleic Acids Res. 12:369-85); the potatoproteinase inhibitor II (PINII) gene (Keil, et al., (1986) Nucleic AcidsRes. 14:5641-50 and An, et al., (1989) Plant Cell 1:115-22) and the CaMV19S gene (Mogen, et al., (1990) Plant Cell 2:1261-72).

An intron sequence can be added to the 5′ untranslated region or thecoding sequence of the partial coding sequence to increase the amount ofthe mature message that accumulates in the cytosol. Inclusion of aspliceable intron in the transcription unit in both plant and animalexpression constructs has been shown to increase gene expression at boththe mRNA and protein levels up to 1000-fold (Buchman and Berg, (1988)Mol. Cell Biol. 8:4395-4405; Callis, et al., (1987) Genes Dev.1:1183-200). Such intron enhancement of gene expression is typicallygreatest when placed near the 5′ end of the transcription unit. Use ofmaize introns Adh1-S intron 1, 2 and 6, the Bronze-1 intron are known inthe art. See generally, The Maize Handbook, Chapter 116, Freeling andWalbot, eds., Springer, New York (1994).

Plant signal sequences, including, but not limited to, signal-peptideencoding DNA/RNA sequences which target proteins to the extracellularmatrix of the plant cell (Dratewka-Kos, et al., (1989) J. Biol. Chem.264:4896-900), such as the Nicotiana plumbaginifolia extension gene(DeLoose, et al., (1991) Gene 99:95-100); signal peptides which targetproteins to the vacuole, such as the sweet potato sporamin gene(Matsuka, et al., (1991) Proc. Natl. Acad. Sci. USA 88:834) and thebarley lectin gene (Wilkins, et al., (1990) Plant Cell, 2:301-13);signal peptides which cause proteins to be secreted, such as that ofPRlb (Lind, et al., (1992) Plant Mol. Biol. 18:47-53) or the barleyalpha amylase (BAA) (Rahmatullah, et al., (1989) Plant Mol. Biol.12:119, and hereby incorporated by reference) or signal peptides whichtarget proteins to the plastids such as that of rapeseed enoyl-Acpreductase (Verwaert, et al., (1994) Plant Mol. Biol. 26:189-202) areuseful in the invention.

The vector comprising the sequences from a polynucleotide of the presentinvention will typically comprise a marker gene, which confers aselectable phenotype on plant cells. Usually, the selectable marker genewill encode antibiotic resistance, with suitable genes including genescoding for resistance to the antibiotic spectinomycin (e.g., the aadagene), the streptomycin phosphotransferase (SPT) gene coding forstreptomycin resistance, the neomycin phosphotransferase (NPTII) geneencoding kanamycin or geneticin resistance, the hygromycinphosphotransferase (HPT) gene coding for hygromycin resistance, genescoding for resistance to herbicides which act to inhibit the action ofacetolactate synthase (ALS), in particular the sulfonylurea-typeherbicides (e.g., the acetolactate synthase (ALS) gene containingmutations leading to such resistance in particular the S4 and/or Hramutations), genes coding for resistance to herbicides which act toinhibit action of glutamine synthase, such as phosphinothricin or basta(e.g., the bar gene) or other such genes known in the art. The bar geneencodes resistance to the herbicide basta and the ALS gene encodesresistance to the herbicide chlorsulfuron.

Typical vectors useful for expression of genes in higher plants are wellknown in the art and include vectors derived from the tumor-inducing(Ti) plasmid of Agrobacterium tumefaciens described by Rogers, et al.,(1987), Meth. Enzymol. 153:253-77. These vectors are plant integratingvectors in that on transformation, the vectors integrate a portion ofvector DNA into the genome of the host plant. Exemplary A. tumefaciensvectors useful herein are plasmids pKYLX6 and pKYLX7 of Schardl, et al.,(1987) Gene 61:1-11 and Berger, et al., (1989) Proc. Natl. Acad. Sci.USA, 86:8402-6. Another useful vector herein is plasmid pBI101.2 that isavailable from CLONTECH Laboratories, Inc. (Palo Alto, Calif.).

Expression of Proteins in Host Cells

Using the nucleic acids of the present invention, one may express aprotein of the present invention in a recombinantly engineered cell suchas bacteria, yeast, insect, mammalian or preferably plant cells. Thecells produce the protein in a non-natural condition (e.g., in quantity,composition, location, and/or time), because they have been geneticallyaltered through human intervention to do so.

It is expected that those of skill in the art are knowledgeable in thenumerous expression systems available for expression of a nucleic acidencoding a protein of the present invention. No attempt to describe indetail the various methods known for the expression of proteins inprokaryotes or eukaryotes will be made.

In brief summary, the expression of isolated nucleic acids encoding aprotein of the present invention will typically be achieved by operablylinking, for example, the DNA or cDNA to a promoter (which is eitherconstitutive or inducible), followed by incorporation into an expressionvector. The vectors can be suitable for replication and integration ineither prokaryotes or eukaryotes. Typical expression vectors containtranscription and translation terminators, initiation sequences, andpromoters useful for regulation of the expression of the DNA encoding aprotein of the present invention. To obtain high level expression of acloned gene, it is desirable to construct expression vectors whichcontain, at the minimum, a strong promoter, such as ubiquitin, to directtranscription, a ribosome binding site for translational initiation anda transcription/translation terminator. Constitutive promoters areclassified as providing for a range of constitutive expression. Thus,some are weak constitutive promoters and others are strong constitutivepromoters. Generally, by “weak promoter” is intended a promoter thatdrives expression of a coding sequence at a low level. By “low level” isintended at levels of about 1/10,000 transcripts to about 1/100,000transcripts to about 1/500,000 transcripts. Conversely, a “strongpromoter” drives expression of a coding sequence at a “high level” orabout 1/10 transcripts to about 1/100 transcripts to about 1/1,000transcripts.

One of skill would recognize that modifications could be made to aprotein of the present invention without diminishing its biologicalactivity. Some modifications may be made to facilitate the cloning,expression, or incorporation of the targeting molecule into a fusionprotein. Such modifications are well known to those of skill in the artand include, for example, a methionine added at the amino terminus toprovide an initiation site, or additional amino acids (e.g., poly His)placed on either terminus to create conveniently located restrictionsites or termination codons or purification sequences.

Expression in Prokaryotes

Prokaryotic cells may be used as hosts for expression. Prokaryotes mostfrequently are represented by various strains of E. coli; however, othermicrobial strains may also be used. Commonly used prokaryotic controlsequences which are defined herein to include promoters fortranscription initiation, optionally with an operator, along withribosome binding site sequences, include such commonly used promoters asthe beta lactamase (penicillinase) and lactose (lac) promoter systems(Chang, et al., (1977) Nature 198:1056), the tryptophan (trp) promotersystem (Goeddel, et al., (1980) Nucleic Acids Res. 8:4057) and thelambda derived P L promoter and N-gene ribosome binding site (Shimatake,et al., (1981) Nature 292:128). The inclusion of selection markers inDNA vectors transfected in E. coli is also useful. Examples of suchmarkers include genes specifying resistance to ampicillin, tetracycline,or chloramphenicol.

The vector is selected to allow introduction of the gene of interestinto the appropriate host cell. Bacterial vectors are typically ofplasmid or phage origin. Appropriate bacterial cells are infected withphage vector particles or transfected with naked phage vector DNA. If aplasmid vector is used, the bacterial cells are transfected with theplasmid vector DNA. Expression systems for expressing a protein of thepresent invention are available using Bacillus sp. and Salmonella(Palva, et al., (1983) Gene 22:229-35; Mosbach, et al., (1983) Nature302:543-5). The pGEX-4T-1 plasmid vector from Pharmacia is the preferredE. coli expression vector for the present invention.

Expression in Eukaryotes

A variety of eukaryotic expression systems such as yeast, insect celllines, plant and mammalian cells, are known to those of skill in theart. As explained briefly below, the present invention can be expressedin these eukaryotic systems. In some embodiments,transformed/transfected plant cells, as discussed infra, are employed asexpression systems for production of the proteins of the instantinvention.

Synthesis of heterologous proteins in yeast is well known. Sherman, etal., (1982) Methods in Yeast Genetics, Cold Spring Harbor Laboratory isa well recognized work describing the various methods available toproduce the protein in yeast. Two widely utilized yeasts for productionof eukaryotic proteins are Saccharomyces cerevisiae and Pichia pastoris.Vectors, strains and protocols for expression in Saccharomyces andPichia are known in the art and available from commercial suppliers(e.g., Invitrogen). Suitable vectors usually have expression controlsequences, such as promoters, including 3-phosphoglycerate kinase oralcohol oxidase and an origin of replication, termination sequences andthe like as desired.

A protein of the present invention, once expressed, can be isolated fromyeast by lysing the cells and applying standard protein isolationtechniques to the lysates or the pellets. The monitoring of thepurification process can be accomplished by using Western blottechniques or radioimmunoassay of other standard immunoassay techniques.

The sequences encoding proteins of the present invention can also beligated to various expression vectors for use in transfecting cellcultures of, for instance, mammalian, insect or plant origin. Mammaliancell systems often will be in the form of monolayers of cells althoughmammalian cell suspensions may also be used. A number of suitable hostcell lines capable of expressing intact proteins have been developed inthe art, and include the HEK293, BHK21, and CHO cell lines. Expressionvectors for these cells can include expression control sequences, suchas an origin of replication, a promoter (e.g., the CMV promoter, a HSVtk promoter or pgk (phosphoglycerate kinase) promoter), an enhancer(Queen, et al., (1986) Immunol. Rev. 89:49) and necessary processinginformation sites, such as ribosome binding sites, RNA splice sites,polyadenylation sites (e.g., an SV40 large T Ag poly A addition site)and transcriptional terminator sequences. Other animal cells useful forproduction of proteins of the present invention are available, forinstance, from the American Type Culture Collection Catalogue of CellLines and Hybridomas (7^(th) ed., 1992).

Appropriate vectors for expressing proteins of the present invention ininsect cells are usually derived from the SF9 baculovirus. Suitableinsect cell lines include mosquito larvae, silkworm, armyworm, moth andDrosophila cell lines such as a Schneider cell line (see, e.g.,Schneider, (1987) J. Embryol. Exp. Morphol. 27:353-65).

As with yeast, when higher animal or plant host cells are employed,polyadenlyation or transcription terminator sequences are typicallyincorporated into the vector. An example of a terminator sequence is thepolyadenlyation sequence from the bovine growth hormone gene. Otheruseful terminators for practicing this invention include, but are notlimited to, pinII (see, An, et al., (1989) Plant Cell 1(1):115-122),glb1 (see, Genbank Accession Number L22345), gz (see, gzw64a terminator,Genbank Accession Number S78780) and the nos terminator fromAgrobacterium.

Sequences for accurate splicing of the transcript may also be included.An example of a splicing sequence is the VP1 intron from SV40 (Sprague,et al., (1983) J. Virol. 45:773-81). Additionally, gene sequences tocontrol replication in the host cell may be incorporated into the vectorsuch as those found in bovine papilloma virus type-vectors(Saveria-Campo, “Bovine Papilloma Virus DNA a Eukaryotic CloningVector,” in DNA Cloning: A Practical Approach, vol. II, Glover, ed., IRLPress, Arlington, Va., pp. 213-38 (1985)).

In addition, the NR gene placed in the appropriate plant expressionvector can be used to transform plant cells. The polypeptide can then beisolated from plant callus or the transformed cells can be used toregenerate transgenic plants. Such transgenic plants can be harvested,and the appropriate tissues (seed or leaves, for example) can besubjected to large scale protein extraction and purification techniques.

Plant Transformation Methods

Numerous methods for introducing foreign genes into plants are known andcan be used to insert an NR polynucleotide into a plant host, includingbiological and physical plant transformation protocols. See, e.g., Miki,et al., (1993) “Procedure for Introducing Foreign DNA into Plants,” inMethods in Plant Molecular Biology and Biotechnology, Glick andThompson, eds., CRC Press, Inc., Boca Raton, pp. 67-88. The methodschosen vary with the host plant and include chemical transfectionmethods such as calcium phosphate, microorganism-mediated gene transfersuch as Agrobacterium (Horsch, et al., (1985) Science 227:1229-31),electroporation, micro-injection and biolistic bombardment. Expressioncassettes and vectors and in vitro culture methods for plant cell ortissue transformation and regeneration of plants are known andavailable. See, e.g., Gruber, et al., “Vectors for PlantTransformation,” in Methods in Plant Molecular Biology andBiotechnology, supra, pp. 89-119.

The isolated polynucleotides or polypeptides may be introduced into theplant by one or more techniques typically used for direct delivery intocells. Such protocols may vary depending on the type of organism, cell,plant or plant cell, i.e. monocot or dicot, targeted for genemodification. Suitable methods of transforming plant cells includemicroinjection (Crossway, et al., (1986) Biotechniques 4:320-334 andU.S. Pat. No. 6,300,543), electroporation (Riggs, et al., (1986) Proc.Natl. Acad. Sci. USA 83:5602-5606, direct gene transfer (Paszkowski, etal., (1984) EMBO J. 3:2717-2722) and ballistic particle acceleration(see, for example, Sanford, et al., U.S. Pat. No. 4,945,050; WO1991/10725 and McCabe, et al., (1988) Biotechnology 6:923-926). Alsosee, Tomes, et al., “Direct DNA Transfer into Intact Plant Cells ViaMicroprojectile Bombardment”. pp. 197-213 in Plant Cell, Tissue andOrgan Culture, Fundamental Methods. eds. Gamborg and Phillips.Springer-Verlag Berlin Heidelberg New York, 1995; U.S. Pat. No.5,736,369 (meristem); Weissinger, et al., (1988) Ann. Rev. Genet.22:421-477; Sanford, et al., (1987) Particulate Science and Technology5:27-37 (onion); Christou, et al., (1988) Plant Physiol. 87:671-674(soybean); Datta, et al., (1990) Biotechnology 8:736-740 (rice); Klein,et al., (1988) Proc. Natl. Acad. Sci. USA 85:4305-4309 (maize); Klein,et al., (1988) Biotechnology 6:559-563 (maize); WO 1991/10725 (maize);Klein, et al., (1988) Plant Physiol. 91:440-444 (maize); Fromm, et al.,(1990) Biotechnology 8:833-839 and Gordon-Kamm, et al., (1990) PlantCell 2:603-618 (maize); Hooydaas-Van Slogteren and Hooykaas, (1984)Nature (London) 311:763-764; Bytebierm, et al., (1987) Proc. Natl. Acad.Sci. USA 84:5345-5349 (Liliaceae); De Wet, et al., (1985) In TheExperimental Manipulation of Ovule Tissues, ed. Chapman, et al., pp.197-209. Longman, N.Y. (pollen); Kaeppler, et al., (1990) Plant CellReports 9:415-418 and Kaeppler, et al., (1992) Theor. Appl. Genet.84:560-566 (whisker-mediated transformation); U.S. Pat. No. 5,693,512(sonication); D'Halluin, et al., (1992) Plant Cell 4:1495-1505(electroporation); Li, et al., (1993) Plant Cell Reports 12:250-255 andChristou and Ford, (1995) Annals of Botany 75:407-413 (rice); Osjoda, etal., (1996) Nature Biotech. 14:745-750; Agrobacterium mediated maizetransformation (U.S. Pat. No. 5,981,840); silicon carbide whiskermethods (Frame, et al., (1994) Plant J. 6:941-948); laser methods (Guo,et al., (1995) Physiologia Plantarum 93:19-24); sonication methods (Bao,et al., (1997) Ultrasound in Medicine & Biology 23:953-959; Finer andFiner, (2000) Lett Appl Microbiol. 30:406-10; Amoah, et al., (2001) JExp Bot 52:1135-42); polyethylene glycol methods (Krens, et al., (1982)Nature 296:72-77); protoplasts of monocot and dicot cells can betransformed using electroporation (Fromm, et al., (1985) Proc. Natl.Acad. Sci. USA 82:5824-5828) and microinjection (Crossway, et al.,(1986) Mol. Gen. Genet. 202:179-185), all of which are hereinincorporated by reference.

Agrobacterium-Mediated Transformation

The most widely utilized method for introducing an expression vectorinto plants is based on the natural transformation system ofAgrobacterium. A. tumefaciens and A. rhizogenes are plant pathogenicsoil bacteria, which genetically transform plant cells. The Ti and Riplasmids of A. tumefaciens and A. rhizogenes, respectively, carry genesresponsible for genetic transformation of plants. See, e.g., Kado,(1991) Crit. Rev. Plant Sci. 10:1. Descriptions of the Agrobacteriumvector systems and methods for Agrobacterium-mediated gene transfer areprovided in Gruber, et al., supra; Miki, et al., supra and Moloney, etal., (1989) Plant Cell Reports 8:238.

Similarly, the gene can be inserted into the T-DNA region of a Ti or Riplasmid derived from A. tumefaciens or A. rhizogenes, respectively.Thus, expression cassettes can be constructed as above, using theseplasmids. Many control sequences are known which when coupled to aheterologous coding sequence and transformed into a host organism showfidelity in gene expression with respect to tissue/organ specificity ofthe original coding sequence. See, e.g., Benfey and Chua, (1989) Science244:174-81. Particularly suitable control sequences for use in theseplasmids are promoters for constitutive leaf-specific expression of thegene in the various target plants. Other useful control sequencesinclude a promoter and terminator from the nopaline synthase gene (NOS).The NOS promoter and terminator are present in the plasmid pARC2,available from the American Type Culture Collection and designated ATCC67238. If such a system is used, the virulence (vir) gene from eitherthe Ti or Ri plasmid must also be present, either along with the T-DNAportion, or via a binary system where the vir gene is present on aseparate vector. Such systems, vectors for use therein, and methods oftransforming plant cells are described in U.S. Pat. No. 4,658,082; U.S.patent application Ser. No. 09/13,914, filed Oct. 1, 1986, as referencedin U.S. Pat. No. 5,262,306, issued Nov. 16, 1993 and Simpson, et al.,(1986) Plant Mol. Biol. 6:403-15 (also referenced in the '306 patent),all incorporated by reference in their entirety.

Once constructed, these plasmids can be placed into A. rhizogenes or A.tumefaciens and these vectors used to transform cells of plant species,which are ordinarily susceptible to Fusarium or Alternaria infection.Several other transgenic plants are also contemplated by the presentinvention including but not limited to soybean, corn, sorghum, alfalfa,rice, clover, cabbage, banana, coffee, celery, tobacco, cowpea, cotton,melon and pepper. The selection of either A. tumefaciens or A.rhizogenes will depend on the plant being transformed thereby. Ingeneral A. tumefaciens is the preferred organism for transformation.Most dicotyledonous plants, some gymnosperms, and a few monocotyledonousplants (e.g., certain members of the Liliales and Arales) aresusceptible to infection with A. tumefaciens. A. rhizogenes also has awide host range, embracing most dicots and some gymnosperms, whichincludes members of the Leguminosae, Compositae and Chenopodiaceae.Monocot plants can now be transformed with some success. European PatentApplication Number 604 662 A1 discloses a method for transformingmonocots using Agrobacterium. European Application Number 672 752 A1discloses a method for transforming monocots with Agrobacterium usingthe scutellum of immature embryos. Ishida, et al., discuss a method fortransforming maize by exposing immature embryos to A. tumefaciens(Nature Biotechnology 14:745-50 (1996)).

Once transformed, these cells can be used to regenerate transgenicplants. For example, whole plants can be infected with these vectors bywounding the plant and then introducing the vector into the wound site.Any part of the plant can be wounded, including leaves, stems and roots.Alternatively, plant tissue, in the form of an explant, such ascotyledonary tissue or leaf disks, can be inoculated with these vectors,and cultured under conditions, which promote plant regeneration. Rootsor shoots transformed by inoculation of plant tissue with A. rhizogenesor A. tumefaciens, containing the gene coding for the fumonisindegradation enzyme, can be used as a source of plant tissue toregenerate fumonisin-resistant transgenic plants, either via somaticembryogenesis or organogenesis. Examples of such methods forregenerating plant tissue are disclosed in Shahin, (1985) Theor. Appl.Genet. 69:235-40; U.S. Pat. No. 4,658,082; Simpson, et al., supra andU.S. patent application Ser. Nos. 09/13,913 and 09/13,914, both filedOct. 1, 1986, as referenced in U.S. Pat. Nos. 5,262,306, issued Nov. 16,1993, the entire disclosures therein incorporated herein by reference.

Direct Gene Transfer

Despite the fact that the host range for Agrobacterium-mediatedtransformation is broad, some major cereal crop species and gymnospermshave generally been recalcitrant to this mode of gene transfer, eventhough some success has recently been achieved in rice (Hiei, et al.,(1994) The Plant Journal 6:271-82). Several methods of planttransformation, collectively referred to as direct gene transfer, havebeen developed as an alternative to Agrobacterium-mediatedtransformation.

A generally applicable method of plant transformation ismicroprojectile-mediated transformation, where DNA is carried on thesurface of microprojectiles measuring about 1 to 4 μm. The expressionvector is introduced into plant tissues with a biolistic device thataccelerates the microprojectiles to speeds of 300 to 600 m/s which issufficient to penetrate the plant cell walls and membranes (Sanford, etal., (1987) Part. Sci. Technol. 5:27; Sanford, (1988) Trends Biotech6:299; Sanford, (1990) Physiol. Plant 79:206 and Klein, et al., (1992)Biotechnology 10:268).

Another method for physical delivery of DNA to plants is sonication oftarget cells as described in Zang, et al., (1991) BioTechnology 9:996.Alternatively, liposome or spheroplast fusions have been used tointroduce expression vectors into plants. See, e.g., Deshayes, et al.,(1985) EMBO J. 4:2731 and Christou, et al., (1987) Proc. Natl. Acad.Sci. USA 84:3962. Direct uptake of DNA into protoplasts using CaCl₂precipitation, polyvinyl alcohol, or poly-L-ornithine has also beenreported. See, e.g., Hain, et al., (1985) Mol. Gen. Genet. 199:161 andDraper, et al., (1982) Plant Cell Physiol. 23:451.

Electroporation of protoplasts and whole cells and tissues has also beendescribed. See, e.g., Donn, et al., (1990) Abstracts of the VIIth Int'l.Congress on Plant Cell and Tissue Culture IAPTC, A2-38, p. 53;D'Halluin, et al., (1992) Plant Cell 4:1495-505 and Spencer, et al.,(1994) Plant Mol. Biol. 24:51-61.

Increasing the Activity and/or Level of a NR Polypeptide

Methods are provided to increase the activity and/or level of the NRpolypeptide of the invention. An increase in the level and/or activityof the NR polypeptide of the invention can be achieved by providing tothe plant a NR polypeptide. The NR polypeptide can be provided byintroducing the amino acid sequence encoding the NR polypeptide into theplant, introducing into the plant a nucleotide sequence encoding a NRpolypeptide or alternatively by modifying a genomic locus encoding theNR polypeptide of the invention.

As discussed elsewhere herein, many methods are known the art forproviding a polypeptide to a plant including, but not limited to, directintroduction of the polypeptide into the plant, introducing into theplant (transiently or stably) a polynucleotide construct encoding apolypeptide having enhanced nitrogen utilization activity. It is alsorecognized that the methods of the invention may employ a polynucleotidethat is not capable of directing, in the transformed plant, theexpression of a protein or an RNA. Thus, the level and/or activity of aNR polypeptide may be increased by altering the gene encoding the NRpolypeptide or its promoter. See, e.g., Kmiec, U.S. Pat. No. 5,565,350;Zarling, et al., PCT/US93/03868. Therefore mutagenized plants that carrymutations in NR genes, where the mutations increase expression of the NRgene or increase the NR activity of the encoded NR polypeptide areprovided.

Reducing the Activity and/or Level of a NR Polypeptide

Methods are provided to reduce or eliminate the activity of a NRpolypeptide of the invention by transforming a plant cell with anexpression cassette that expresses a polynucleotide that inhibits theexpression of the NR polypeptide. The polynucleotide may inhibit theexpression of the NR polypeptide directly, by preventing transcriptionor translation of the NR messenger RNA, or indirectly, by encoding apolypeptide that inhibits the transcription or translation of an NR geneencoding NR polypeptide. Methods for inhibiting or eliminating theexpression of a gene in a plant are well known in the art, and any suchmethod may be used in the present invention to inhibit the expression ofNR polypeptide.

In accordance with the present invention, the expression of NRpolypeptide is inhibited if the protein level of the NR polypeptide isless than 70% of the protein level of the same NR polypeptide in a plantthat has not been genetically modified or mutagenized to inhibit theexpression of that NR polypeptide. In particular embodiments of theinvention, the protein level of the NR polypeptide in a modified plantaccording to the invention is less than 60%, less than 50%, less than40%, less than 30%, less than 20%, less than 10%, less than 5% or lessthan 2% of the protein level of the same NR polypeptide in a plant thatis not a mutant or that has not been genetically modified to inhibit theexpression of that NR polypeptide. The expression level of the NRpolypeptide may be measured directly, for example, by assaying for thelevel of NR polypeptide expressed in the plant cell or plant, orindirectly, for example, by measuring the nitrogen uptake activity ofthe NR polypeptide in the plant cell or plant or by measuring thephenotypic changes in the plant. Methods for performing such assays aredescribed elsewhere herein.

In other embodiments of the invention, the activity of the NRpolypeptides is reduced or eliminated by transforming a plant cell withan expression cassette comprising a polynucleotide encoding apolypeptide that inhibits the activity of a NR polypeptide. The enhancednitrogen utilization activity of a NR polypeptide is inhibited accordingto the present invention if the NR activity of the NR polypeptide isless than 70% of the NR activity of the same NR polypeptide in a plantthat has not been modified to inhibit the NR activity of that NRpolypeptide. In particular embodiments of the invention, the NR activityof the NR polypeptide in a modified plant according to the invention isless than 60%, less than 50%, less than 40%, less than 30%, less than20%, less than 10% or less than 5% of the NR activity of the same NRpolypeptide in a plant that that has not been modified to inhibit theexpression of that NR polypeptide. The NR activity of a NR polypeptideis “eliminated” according to the invention when it is not detectable bythe assay methods described elsewhere herein. Methods of determining thealteration of nitrogen utilization activity of a NR polypeptide aredescribed elsewhere herein.

In other embodiments, the activity of a NR polypeptide may be reduced oreliminated by disrupting the gene encoding the NR polypeptide. Theinvention encompasses mutagenized plants that carry mutations in NRgenes, where the mutations reduce expression of the NR gene or inhibitthe nitrogen utilization activity of the encoded NR polypeptide.

Thus, many methods may be used to reduce or eliminate the activity of aNR polypeptide. In addition, more than one method may be used to reducethe activity of a single NR polypeptide.

1. Polynucleotide-Based Methods:

In some embodiments of the present invention, a plant is transformedwith an expression cassette that is capable of expressing apolynucleotide that inhibits the expression of an NR polypeptide of theinvention. The term “expression” as used herein refers to thebiosynthesis of a gene product, including the transcription and/ortranslation of said gene product. For example, for the purposes of thepresent invention, an expression cassette capable of expressing apolynucleotide that inhibits the expression of at least one NRpolypeptide is an expression cassette capable of producing an RNAmolecule that inhibits the transcription and/or translation of at leastone NR polypeptide of the invention. The “expression” or “production” ofa protein or polypeptide from a DNA molecule refers to the transcriptionand translation of the coding sequence to produce the protein orpolypeptide, while the “expression” or “production” of a protein orpolypeptide from an RNA molecule refers to the translation of the RNAcoding sequence to produce the protein or polypeptide.

Examples of polynucleotides that inhibit the expression of a NRpolypeptide are given below.

i. Sense Suppression/Cosuppression

In some embodiments of the invention, inhibition of the expression of aNR polypeptide may be obtained by sense suppression or cosuppression.For cosuppression, an expression cassette is designed to express an RNAmolecule corresponding to all or part of a messenger RNA encoding a NRpolypeptide in the “sense” orientation. Over expression of the RNAmolecule can result in reduced expression of the native gene.Accordingly, multiple plant lines transformed with the cosuppressionexpression cassette are screened to identify those that show thegreatest inhibition of NR polypeptide expression.

The polynucleotide used for cosuppression may correspond to all or partof the sequence encoding the NR polypeptide, all or part of the 5′and/or 3′ untranslated region of a NR polypeptide transcript or all orpart of both the coding sequence and the untranslated regions of atranscript encoding a NR polypeptide. In some embodiments where thepolynucleotide comprises all or part of the coding region for the NRpolypeptide, the expression cassette is designed to eliminate the startcodon of the polynucleotide so that no protein product will betranslated.

Cosuppression may be used to inhibit the expression of plant genes toproduce plants having undetectable protein levels for the proteinsencoded by these genes. See, for example, Broin, et al., (2002) PlantCell 14:1417-1432. Cosuppression may also be used to inhibit theexpression of multiple proteins in the same plant. See, for example,U.S. Pat. No. 5,942,657. Methods for using cosuppression to inhibit theexpression of endogenous genes in plants are described in Flavell, etal., (1994) Proc. Natl. Acad. Sci. USA 91:3490-3496; Jorgensen, et al.,(1996) Plant Mol. Biol. 31:957-973; Johansen and Carrington, (2001)Plant Physiol. 126:930-938; Broin, et al., (2002) Plant Cell14:1417-1432; Stoutjesdijk, et al., (2002) Plant Physiol. 129:1723-1731;Yu, et al., (2003) Phytochemistry 63:753-763 and U.S. Pat. Nos.5,034,323, 5,283,184 and 5,942,657; each of which is herein incorporatedby reference. The efficiency of cosuppression may be increased byincluding a poly-dT region in the expression cassette at a position 3′to the sense sequence and 5′ of the polyadenylation signal. See, USPatent Application Publication Number 2002/0048814, herein incorporatedby reference. Typically, such a nucleotide sequence has substantialsequence identity to the sequence of the transcript of the endogenousgene, optimally greater than about 65% sequence identity, more optimallygreater than about 85% sequence identity, most optimally greater thanabout 95% sequence identity. See, U.S. Pat. Nos. 5,283,184 and5,034,323, herein incorporated by reference.

ii. Antisense Suppression

In some embodiments of the invention, inhibition of the expression ofthe NR polypeptide may be obtained by antisense suppression. Forantisense suppression, the expression cassette is designed to express anRNA molecule complementary to all or part of a messenger RNA encodingthe NR polypeptide. Over expression of the antisense RNA molecule canresult in reduced expression of the native gene. Accordingly, multipleplant lines transformed with the antisense suppression expressioncassette are screened to identify those that show the greatestinhibition of NR polypeptide expression.

The polynucleotide for use in antisense suppression may correspond toall or part of the complement of the sequence encoding the NRpolypeptide, all or part of the complement of the 5′ and/or 3′untranslated region of the NR transcript or all or part of thecomplement of both the coding sequence and the untranslated regions of atranscript encoding the NR polypeptide. In addition, the antisensepolynucleotide may be fully complementary (i.e., 100% identical to thecomplement of the target sequence) or partially complementary (i.e.,less than 100% identical to the complement of the target sequence) tothe target sequence. Antisense suppression may be used to inhibit theexpression of multiple proteins in the same plant. See, for example,U.S. Pat. No. 5,942,657. Furthermore, portions of the antisensenucleotides may be used to disrupt the expression of the target gene.Generally, sequences of at least 50 nucleotides, 100 nucleotides, 200nucleotides, 300, 400, 450, 500, 550 or greater may be used. Methods forusing antisense suppression to inhibit the expression of endogenousgenes in plants are described, for example, in Liu, et al., (2002) PlantPhysiol. 129:1732-1743 and U.S. Pat. Nos. 5,759,829 and 5,942,657, eachof which is herein incorporated by reference.

Efficiency of antisense suppression may be increased by including apoly-dT region in the expression cassette at a position 3′ to theantisense sequence and 5′ of the polyadenylation signal. See, US PatentApplication Publication Number 2002/0048814, herein incorporated byreference.

iii. Double-Stranded RNA Interference

In some embodiments of the invention, inhibition of the expression of aNR polypeptide may be obtained by double-stranded RNA (dsRNA)interference. For dsRNA interference, a sense RNA molecule like thatdescribed above for cosuppression and an antisense RNA molecule that isfully or partially complementary to the sense RNA molecule are expressedin the same cell, resulting in inhibition of the expression of thecorresponding endogenous messenger RNA.

Expression of the sense and antisense molecules can be accomplished bydesigning the expression cassette to comprise both a sense sequence andan antisense sequence. Alternatively, separate expression cassettes maybe used for the sense and antisense sequences. Multiple plant linestransformed with the dsRNA interference expression cassette orexpression cassettes are then screened to identify plant lines that showthe greatest inhibition of NR polypeptide expression. Methods for usingdsRNA interference to inhibit the expression of endogenous plant genesare described in Waterhouse, et al., (1998) Proc. Natl. Acad. Sci. USA95:13959-13964, Liu, et al., (2002) Plant Physiol. 129:1732-1743, and WO1999/49029, WO 1999/53050, WO 1999/61631 and WO 2000/49035, each ofwhich is herein incorporated by reference.

iv. Hairpin RNA Interference and Intron-Containing Hairpin RNAInterference

In some embodiments of the invention, inhibition of the expression of aNR polypeptide may be obtained by hairpin RNA (hpRNA) interference orintron-containing hairpin RNA (ihpRNA) interference. These methods arehighly efficient at inhibiting the expression of endogenous genes. See,Waterhouse and Helliwell, (2003) Nat. Rev. Genet. 4:29-38 and thereferences cited therein.

For hpRNA interference, the expression cassette is designed to expressan RNA molecule that hybridizes with itself to form a hairpin structurethat comprises a single-stranded loop region and a base-paired stem. Thebase-paired stem region comprises a sense sequence corresponding to allor part of the endogenous messenger RNA encoding the gene whoseexpression is to be inhibited and an antisense sequence that is fully orpartially complementary to the sense sequence. Alternatively, thebase-paired stem region may correspond to a portion of a promotersequence controlling expression of the gene to be inhibited. Thus, thebase-paired stem region of the molecule generally determines thespecificity of the RNA interference. hpRNA molecules are highlyefficient at inhibiting the expression of endogenous genes and the RNAinterference they induce is inherited by subsequent generations ofplants. See, for example, Chuang and Meyerowitz, (2000) Proc. Natl.Acad. Sci. USA 97:4985-4990; Stoutjesdijk, et al., (2002) Plant Physiol.129:1723-1731 and Waterhouse and Helliwell, (2003) Nat. Rev. Genet.4:29-38. Methods for using hpRNA interference to inhibit or silence theexpression of genes are described, for example, in Chuang andMeyerowitz, (2000) Proc. Natl. Acad. Sci. USA 97:4985-4990;Stoutjesdijk, et al., (2002) Plant Physiol. 129:1723-1731; Waterhouseand Helliwell, (2003) Nat. Rev. Genet. 4:29-38; Pandolfini et al., BMCBiotechnology 3:7 and US Patent Publication Number 2003/0175965, each ofwhich is herein incorporated by reference. A transient assay for theefficiency of hpRNA constructs to silence gene expression in vivo hasbeen described by Panstruga, et al., (2003) Mol. Biol. Rep. 30:135-140,herein incorporated by reference.

For ihpRNA, the interfering molecules have the same general structure asfor hpRNA, but the RNA molecule additionally comprises an intron that iscapable of being spliced in the cell in which the ihpRNA is expressed.The use of an intron minimizes the size of the loop in the hairpin RNAmolecule following splicing, and this increases the efficiency ofinterference. See, for example, Smith, et al., (2000) Nature407:319-320. In fact, Smith, et al., show 100% suppression of endogenousgene expression using ihpRNA-mediated interference. Methods for usingihpRNA interference to inhibit the expression of endogenous plant genesare described, for example, in Smith, et al., (2000) Nature 407:319-320;Wesley, et al., (2001) Plant J. 27:581-590; Wang and Waterhouse, (2001)Curr. Opin. Plant Biol. 5:146-150; Waterhouse and Helliwell, (2003) Nat.Rev. Genet. 4:29-38; Helliwell and Waterhouse, (2003) Methods 30:289-295and US Patent Publication Number 2003/0180945, each of which is hereinincorporated by reference.

The expression cassette for hpRNA interference may also be designed suchthat the sense sequence and the antisense sequence do not correspond toan endogenous RNA. In this embodiment, the sense and antisense sequenceflank a loop sequence that comprises a nucleotide sequence correspondingto all or part of the endogenous messenger RNA of the target gene. Thus,it is the loop region that determines the specificity of the RNAinterference. See, for example, WO 2002/00904; Mette, et al., (2000)EMBO J. 19:5194-5201; Matzke, et al., (2001) Curr. Opin. Genet. Devel.11:221-227; Scheid, et al., (2002) Proc. Natl. Acad. Sci., USA99:13659-13662; Aufsaftz, et al., (2002) Proc. Nat'l. Acad. Sci.99(4):16499-16506; Sijen, et al., Curr. Biol. (2001) 11:436-440), hereinincorporated by reference.

v. Amplicon-Mediated Interference

Amplicon expression cassettes comprise a plant virus-derived sequencethat contains all or part of the target gene but generally not all ofthe genes of the native virus. The viral sequences present in thetranscription product of the expression cassette allow the transcriptionproduct to direct its own replication. The transcripts produced by theamplicon may be either sense or antisense relative to the targetsequence (i.e., the messenger RNA for the NR polypeptide). Methods ofusing amplicons to inhibit the expression of endogenous plant genes aredescribed, for example, in Angell and Baulcombe, (1997) EMBO J.16:3675-3684, Angell and Baulcombe, (1999) Plant J. 20:357-362 and U.S.Pat. No. 6,646,805, each of which is herein incorporated by reference.

vi. Ribozymes

In some embodiments, the polynucleotide expressed by the expressioncassette of the invention is catalytic RNA or has ribozyme activityspecific for the messenger RNA of the NR polypeptide. Thus, thepolynucleotide causes the degradation of the endogenous messenger RNA,resulting in reduced expression of the NR polypeptide. This method isdescribed, for example, in U.S. Pat. No. 4,987,071, herein incorporatedby reference.

vii. Small Interfering RNA or Micro RNA

In some embodiments of the invention, inhibition of the expression of aNR polypeptide may be obtained by RNA interference by expression of agene encoding a micro RNA (miRNA). miRNAs are regulatory agentsconsisting of about 22 ribonucleotides. miRNA are highly efficient atinhibiting the expression of endogenous genes. See, for example Javier,et al., (2003) Nature 425:257-263, herein incorporated by reference.

For miRNA interference, the expression cassette is designed to expressan RNA molecule that is modeled on an endogenous miRNA gene. The miRNAgene encodes an RNA that forms a hairpin structure containing a22-nucleotide sequence that is complementary to another endogenous gene(target sequence). For suppression of NR expression, the 22-nucleotidesequence is selected from a NR transcript sequence and contains 22nucleotides of said NR sequence in sense orientation and 21 nucleotidesof a corresponding antisense sequence that is complementary to the sensesequence. miRNA molecules are highly efficient at inhibiting theexpression of endogenous genes, and the RNA interference they induce isinherited by subsequent generations of plants.

2. Polypeptide-Based Inhibition of Gene Expression

In one embodiment, the polynucleotide encodes a zinc finger protein thatbinds to a gene encoding a NR polypeptide, resulting in reducedexpression of the gene. In particular embodiments, the zinc fingerprotein binds to a regulatory region of a NR gene. In other embodiments,the zinc finger protein binds to a messenger RNA encoding a NRpolypeptide and prevents its translation. Methods of selecting sites fortargeting by zinc finger proteins have been described, for example, inU.S. Pat. No. 6,453,242 and methods for using zinc finger proteins toinhibit the expression of genes in plants are described, for example, inUS Patent Application Publication Number 2003/0037355, each of which isherein incorporated by reference.

3. Polypeptide-Based Inhibition of Protein Activity

In some embodiments of the invention, the polynucleotide encodes anantibody that binds to at least one NR polypeptide and reduces theenhanced nitrogen utilization activity of the NR polypeptide. In anotherembodiment, the binding of the antibody results in increased turnover ofthe antibody-NR complex by cellular quality control mechanisms. Theexpression of antibodies in plant cells and the inhibition of molecularpathways by expression and binding of antibodies to proteins in plantcells are well known in the art. See, for example, Conrad and Sonnewald,(2003) Nature Biotech. 21:35-36, incorporated herein by reference.

4. Gene Disruption

In some embodiments of the present invention, the activity of a NRpolypeptide is reduced or eliminated by disrupting the gene encoding theNR polypeptide. The gene encoding the NR polypeptide may be disrupted byany method known in the art. For example, in one embodiment, the gene isdisrupted by transposon tagging. In another embodiment, the gene isdisrupted by mutagenizing plants using random or targeted mutagenesisand selecting for plants that have reduced nitrogen utilizationactivity.

i. Transposon Tagging

In one embodiment of the invention, transposon tagging is used to reduceor eliminate the NR activity of one or more NR polypeptide. Transposontagging comprises inserting a transposon within an endogenous NR gene toreduce or eliminate expression of the NR polypeptide. “NR gene” isintended to mean the gene that encodes a NR polypeptide according to theinvention.

In this embodiment, the expression of one or more NR polypeptide isreduced or eliminated by inserting a transposon within a regulatoryregion or coding region of the gene encoding the NR polypeptide. Atransposon that is within an exon, intron, 5′ or 3′ untranslatedsequence, a promoter, or any other regulatory sequence of a NR gene maybe used to reduce or eliminate the expression and/or activity of theencoded NR polypeptide.

Methods for the transposon tagging of specific genes in plants are wellknown in the art. See, for example, Maes, et al., (1999) Trends PlantSci. 4:90-96; Dharmapuri and Sonti, (1999) FEMS Microbiol. Lett.179:53-59; Meissner, et al., (2000) Plant J. 22:265-274; Phogat, et al.,(2000) J. Biosci. 25:57-63; Walbot, (2000) Curr. Opin. Plant Biol.2:103-107; Gai, et al., (2000) Nucleic Acids Res. 28:94-96; Fitzmaurice,et al., (1999) Genetics 153:1919-1928). In addition, the TUSC processfor selecting Mu insertions in selected genes has been described inBensen, et al., (1995) Plant Cell 7:75-84; Mena, et al., (1996) Science274:1537-1540 and U.S. Pat. No. 5,962,764, each of which is hereinincorporated by reference.

ii. Mutant Plants with Reduced Activity

Additional methods for decreasing or eliminating the expression ofendogenous genes in plants are also known in the art and can besimilarly applied to the instant invention. These methods include otherforms of mutagenesis, such as ethyl methanesulfonate-inducedmutagenesis, deletion mutagenesis and fast neutron deletion mutagenesisused in a reverse genetics sense (with PCR) to identify plant lines inwhich the endogenous gene has been deleted. For examples of thesemethods see, Ohshima, et al., (1998) Virology 243:472-481; Okubara, etal., (1994) Genetics 137:867-874 and Quesada, et al., (2000) Genetics154:421-436, each of which is herein incorporated by reference. Inaddition, a fast and automatable method for screening for chemicallyinduced mutations, TILLING (Targeting Induced Local Lesions In Genomes),using denaturing HPLC or selective endonuclease digestion of selectedPCR products is also applicable to the instant invention. See, McCallum,et al., (2000) Nat. Biotechnol. 18:455-457, herein incorporated byreference.

Mutations that impact gene expression or that interfere with thefunction (enhanced nitrogen utilization activity) of the encoded proteinare well known in the art. Insertional mutations in gene exons usuallyresult in null-mutants. Mutations in conserved residues are particularlyeffective in inhibiting the activity of the encoded protein. Conservedresidues of plant NR polypeptides suitable for mutagenesis with the goalto eliminate NR activity have been described. Such mutants can beisolated according to well-known procedures, and mutations in differentNR loci can be stacked by genetic crossing. See, for example, Gruis, etal., (2002) Plant Cell 14:2863-2882.

In another embodiment of this invention, dominant mutants can be used totrigger RNA silencing due to gene inversion and recombination of aduplicated gene locus. See, for example, Kusaba, et al., (2003) PlantCell 15:1455-1467.

The invention encompasses additional methods for reducing or eliminatingthe activity of one or more NR polypeptide. Examples of other methodsfor altering or mutating a genomic nucleotide sequence in a plant areknown in the art and include, but are not limited to, the use of RNA:DNAvectors, RNA:DNA mutational vectors, RNA:DNA repair vectors,mixed-duplex oligonucleotides, self-complementary RNA:DNAoligonucleotides and recombinogenic oligonucleobases. Such vectors andmethods of use are known in the art. See, for example, U.S. Pat. Nos.5,565,350; 5,731,181; 5,756,325; 5,760,012; 5,795,972 and 5,871,984,each of which are herein incorporated by reference. See also, WO1998/49350, WO 1999/07865, WO 1999/25821 and Beetham, et al., (1999)Proc. Natl. Acad. Sci. USA 96:8774-8778, each of which is hereinincorporated by reference.

iii. Modulating Nitrogen Utilization Activity

In specific methods, the level and/or activity of a NR regulator in aplant is decreased by increasing the level or activity of the NRpolypeptide in the plant. The increased expression of a negativeregulatory molecule may decrease the level of expression of downstreamone or more genes responsible for an improved NR phenotype.

Methods for increasing the level and/or activity of NR polypeptides in aplant are discussed elsewhere herein. Briefly, such methods compriseproviding a NR polypeptide of the invention to a plant and therebyincreasing the level and/or activity of the NR polypeptide. In otherembodiments, a NR nucleotide sequence encoding a NR polypeptide can beprovided by introducing into the plant a polynucleotide comprising a NRnucleotide sequence of the invention, expressing the NR sequence,increasing the activity of the NR polypeptide and thereby decreasing thenumber of tissue cells in the plant or plant part. In other embodiments,the NR nucleotide construct introduced into the plant is stablyincorporated into the genome of the plant.

In other methods, the growth of a plant tissue is increased bydecreasing the level and/or activity of the NR polypeptide in the plant.Such methods are disclosed in detail elsewhere herein. In one suchmethod, a NR nucleotide sequence is introduced into the plant andexpression of said NR nucleotide sequence decreases the activity of theNR polypeptide and thereby increasing the tissue growth in the plant orplant part. In other embodiments, the NR nucleotide construct introducedinto the plant is stably incorporated into the genome of the plant.

As discussed above, one of skill will recognize the appropriate promoterto use to modulate the level/activity of a NR in the plant. Exemplarypromoters for this embodiment have been disclosed elsewhere herein.

In other embodiments, such plants have stably incorporated into theirgenome a nucleic acid molecule comprising a NR nucleotide sequence ofthe invention operably linked to a promoter that drives expression inthe plant cell.

iv. Modulating Root Development

Methods for modulating root development in a plant are provided. By“modulating root development” is intended any alteration in thedevelopment of the plant root when compared to a control plant. Suchalterations in root development include, but are not limited to,alterations in the growth rate of the primary root, the fresh rootweight, the extent of lateral and adventitious root formation, thevasculature system, meristem development or radial expansion.

Methods for modulating root development in a plant are provided. Themethods comprise modulating the level and/or activity of the NRpolypeptide in the plant. In one method, a NR sequence of the inventionis provided to the plant. In another method, the NR nucleotide sequenceis provided by introducing into the plant a polynucleotide comprising aNR nucleotide sequence of the invention, expressing the NR sequence andthereby modifying root development. In still other methods, the NRnucleotide construct introduced into the plant is stably incorporatedinto the genome of the plant.

In other methods, root development is modulated by altering the level oractivity of the NR polypeptide in the plant. A change in NR activity canresult in at least one or more of the following alterations to rootdevelopment, including, but not limited to, alterations in root biomassand length.

As used herein, “root growth” encompasses all aspects of growth of thedifferent parts that make up the root system at different stages of itsdevelopment in both monocotyledonous and dicotyledonous plants. It is tobe understood that enhanced root growth can result from enhanced growthof one or more of its parts including the primary root, lateral roots,adventitious roots, etc.

Methods of measuring such developmental alterations in the root systemare known in the art. See, for example, US Patent ApplicationPublication Number 2003/0074698 and Werner, et al., (2001) PNAS18:10487-10492, both of which are herein incorporated by reference.

As discussed above, one of skill will recognize the appropriate promoterto use to modulate root development in the plant. Exemplary promotersfor this embodiment include constitutive promoters and root-preferredpromoters. Exemplary root-preferred promoters have been disclosedelsewhere herein.

Stimulating root growth and increasing root mass by decreasing theactivity and/or level of the NR polypeptide also finds use in improvingthe standability of a plant. The term “resistance to lodging” or“standability” refers to the ability of a plant to fix itself to thesoil. For plants with an erect or semi-erect growth habit, this termalso refers to the ability to maintain an upright position under adverse(environmental) conditions. This trait relates to the size, depth andmorphology of the root system. In addition, stimulating root growth andincreasing root mass by altering the level and/or activity of the NRpolypeptide also finds use in promoting in vitro propagation ofexplants.

Furthermore, higher root biomass production due to NR activity has adirect effect on the yield and an indirect effect of production ofcompounds produced by root cells or transgenic root cells or cellcultures of said transgenic root cells. One example of an interestingcompound produced in root cultures is shikonin, the yield of which canbe advantageously enhanced by said methods.

Accordingly, the present invention further provides plants havingmodulated root development when compared to the root development of acontrol plant. In some embodiments, the plant of the invention has anincreased level/activity of the NR polypeptide of the invention and hasenhanced root growth and/or root biomass. In other embodiments, suchplants have stably incorporated into their genome a nucleic acidmolecule comprising a NR nucleotide sequence of the invention operablylinked to a promoter that drives expression in the plant cell.

v. Modulating Shoot and Leaf Development

Methods are also provided for modulating shoot and leaf development in aplant. By “modulating shoot and/or leaf development” is intended anyalteration in the development of the plant shoot and/or leaf. Suchalterations in shoot and/or leaf development include, but are notlimited to, alterations in shoot meristem development, in leaf number,leaf size, leaf and stem vasculature, internode length and leafsenescence. As used herein, “leaf development” and “shoot development”encompasses all aspects of growth of the different parts that make upthe leaf system and the shoot system, respectively, at different stagesof their development, both in monocotyledonous and dicotyledonousplants. Methods for measuring such developmental alterations in theshoot and leaf system are known in the art. See, for example, Werner, etal., (2001) PNAS 98:10487-10492 and US Patent Application PublicationNumber 2003/0074698, each of which is herein incorporated by reference.

The method for modulating shoot and/or leaf development in a plantcomprises modulating the activity and/or level of a NR polypeptide ofthe invention. In one embodiment, a NR sequence of the invention isprovided. In other embodiments, the NR nucleotide sequence can beprovided by introducing into the plant a polynucleotide comprising a NRnucleotide sequence of the invention, expressing the NR sequence, andthereby modifying shoot and/or leaf development. In other embodiments,the NR nucleotide construct introduced into the plant is stablyincorporated into the genome of the plant.

In specific embodiments, shoot or leaf development is modulated byaltering the level and/or activity of the NR polypeptide in the plant. Achange in NR activity can result in at least one or more of thefollowing alterations in shoot and/or leaf development, including, butnot limited to, changes in leaf number, altered leaf surface, alteredvasculature, internodes and plant growth and alterations in leafsenescence, when compared to a control plant.

As discussed above, one of skill will recognize the appropriate promoterto use to modulate shoot and leaf development of the plant. Exemplarypromoters for this embodiment include constitutive promoters,shoot-preferred promoters, shoot meristem-preferred promoters andleaf-preferred promoters. Exemplary promoters have been disclosedelsewhere herein.

Increasing NR activity and/or level in a plant results in alteredinternodes and growth. Thus, the methods of the invention find use inproducing modified plants. In addition, as discussed above, NR activityin the plant modulates both root and shoot growth. Thus, the presentinvention further provides methods for altering the root/shoot ratio.Shoot or leaf development can further be modulated by altering the leveland/or activity of the NR polypeptide in the plant.

Accordingly, the present invention further provides plants havingmodulated shoot and/or leaf development when compared to a controlplant. In some embodiments, the plant of the invention has an increasedlevel/activity of the NR polypeptide of the invention. In otherembodiments, the plant of the invention has a decreased level/activityof the NR polypeptide of the invention.

vi. Modulating Reproductive Tissue Development

Methods for modulating reproductive tissue development are provided. Inone embodiment, methods are provided to modulate floral development in aplant. By “modulating floral development” is intended any alteration ina structure of a plant's reproductive tissue as compared to a controlplant in which the activity or level of the NR polypeptide has not beenmodulated. “Modulating floral development” further includes anyalteration in the timing of the development of a plant's reproductivetissue (i.e., a delayed or a accelerated timing of floral development)when compared to a control plant in which the activity or level of theNR polypeptide has not been modulated. Macroscopic alterations mayinclude changes in size, shape, number or location of reproductiveorgans, the developmental time period that these structures form or theability to maintain or proceed through the flowering process in times ofenvironmental stress. Microscopic alterations may include changes to thetypes or shapes of cells that make up the reproductive organs.

The method for modulating floral development in a plant comprisesmodulating NR activity in a plant. In one method, a NR sequence of theinvention is provided. A NR nucleotide sequence can be provided byintroducing into the plant a polynucleotide comprising a NR nucleotidesequence of the invention, expressing the NR sequence, and therebymodifying floral development. In other embodiments, the NR nucleotideconstruct introduced into the plant is stably incorporated into thegenome of the plant.

In specific methods, floral development is modulated by increasing thelevel or activity of the NR polypeptide in the plant. A change in NRactivity can result in at least one or more of the following alterationsin floral development, including, but not limited to, altered flowering,changed number of flowers, modified male sterility and altered seed set,when compared to a control plant. Inducing delayed flowering orinhibiting flowering can be used to enhance yield in forage crops suchas alfalfa. Methods for measuring such developmental alterations infloral development are known in the art. See, for example, Mouradov, etal., (2002) The Plant Cell S111-S130, herein incorporated by reference.

As discussed above, one of skill will recognize the appropriate promoterto use to modulate floral development of the plant. Exemplary promotersfor this embodiment include constitutive promoters, inducible promoters,shoot-preferred promoters and inflorescence-preferred promoters.

In other methods, floral development is modulated by altering the leveland/or activity of the NR sequence of the invention. Such methods cancomprise introducing a NR nucleotide sequence into the plant andchanging the activity of the NR polypeptide. In other methods, the NRnucleotide construct introduced into the plant is stably incorporatedinto the genome of the plant. Altering expression of the NR sequence ofthe invention can modulate floral development during periods of stress.Such methods are described elsewhere herein. Accordingly, the presentinvention further provides plants having modulated floral developmentwhen compared to the floral development of a control plant. Compositionsinclude plants having a altered level/activity of the NR polypeptide ofthe invention and having an altered floral development. Compositionsalso include plants having a modified level/activity of the NRpolypeptide of the invention wherein the plant maintains or proceedsthrough the flowering process in times of stress.

Methods are also provided for the use of the NR sequences of theinvention to increase seed size and/or weight. The method comprisesincreasing the activity of the NR sequences in a plant or plant part,such as the seed. An increase in seed size and/or weight comprises anincreased size or weight of the seed and/or an increase in the size orweight of one or more seed part including, for example, the embryo,endosperm, seed coat, aleurone or cotyledon.

As discussed above, one of skill will recognize the appropriate promoterto use to increase seed size and/or seed weight. Exemplary promoters ofthis embodiment include constitutive promoters, inducible promoters,seed-preferred promoters, embryo-preferred promoters andendosperm-preferred promoters.

The method for altering seed size and/or seed weight in a plantcomprises increasing NR activity in the plant. In one embodiment, the NRnucleotide sequence can be provided by introducing into the plant apolynucleotide comprising a NR nucleotide sequence of the invention,expressing the NR sequence and thereby decreasing seed weight and/orsize. In other embodiments, the NR nucleotide construct introduced intothe plant is stably incorporated into the genome of the plant.

It is further recognized that increasing seed size and/or weight canalso be accompanied by an increase in the speed of growth of seedlingsor an increase in early vigor. As used herein, the term “early vigor”refers to the ability of a plant to grow rapidly during earlydevelopment, and relates to the successful establishment, aftergermination, of a well-developed root system and a well-developedphotosynthetic apparatus. In addition, an increase in seed size and/orweight can also result in an increase in plant yield when compared to acontrol.

Accordingly, the present invention further provides plants having anincreased seed weight and/or seed size when compared to a control plant.In other embodiments, plants having an increased vigor and plant yieldare also provided. In some embodiments, the plant of the invention has amodified level/activity of the NR polypeptide of the invention and hasan increased seed weight and/or seed size. In other embodiments, suchplants have stably incorporated into their genome a nucleic acidmolecule comprising a NR nucleotide sequence of the invention operablylinked to a promoter that drives expression in the plant cell.

vii. Method of Use for NR Polynucleotide, Expression Cassettes, andAdditional Polynucleotides

The nucleotides, expression cassettes and methods disclosed herein areuseful in regulating expression of any heterologous nucleotide sequencein a host plant in order to vary the phenotype of a plant. Variouschanges in phenotype are of interest including modifying the fatty acidcomposition in a plant, altering the amino acid content of a plant,altering a plant's pathogen defense mechanism and the like. Theseresults can be achieved by providing expression of heterologous productsor increased expression of endogenous products in plants. Alternatively,the results can be achieved by providing for a reduction of expressionof one or more endogenous products, particularly enzymes or cofactors inthe plant. These changes result in a change in phenotype of thetransformed plant.

Genes of interest are reflective of the commercial markets and interestsof those involved in the development of the crop. Crops and markets ofinterest change, and as developing nations open up world markets, newcrops and technologies will emerge also. In addition, as ourunderstanding of agronomic traits and characteristics such as yield andheterosis increase, the choice of genes for transformation will changeaccordingly. General categories of genes of interest include, forexample, those genes involved in information, such as zinc fingers,those involved in communication, such as kinases, and those involved inhousekeeping, such as heat shock proteins. More specific categories oftransgenes, for example, include genes encoding important traits foragronomics, insect resistance, disease resistance, herbicide resistance,sterility, grain characteristics and commercial products. Genes ofinterest include, generally, those involved in oil, starch, carbohydrateor nutrient metabolism as well as those affecting kernel size, sucroseloading and the like.

The polynucleotides of the present invention may be stacked with anygene or combination of genes to produce plants with a variety of desiredtrait combinations, including but not limited to traits desirable foranimal feed such as high oil genes (e.g., U.S. Pat. No. 6,232,529);balanced amino acids (e.g., hordothionins (U.S. Pat. Nos. 5,990,389;5,885,801; 5,885,802 and 5,703,409); barley high lysine (Williamson, etal., (1987) Eur. J. Biochem. 165:99-106 and WO 1998/20122) and highmethionine proteins (Pedersen, et al., (1986) J. Biol. Chem. 261:6279;Kirihara, et al., (1988) Gene 71:359 and Musumura, et al., (1989) PlantMol. Biol. 12:123)); increased digestibility (e.g., modified storageproteins (U.S. patent application Ser. No. 10/053,410, filed Nov. 7,2001) and thioredoxins (U.S. patent application Ser. No. 10/005,429,filed Dec. 3, 2001)), the disclosures of which are herein incorporatedby reference. The polynucleotides of the present invention can also bestacked with traits desirable for insect, disease or herbicideresistance (e.g., Bacillus thuringiensis toxic proteins (U.S. Pat. Nos.5,366,892; 5,747,450; 5,737,514; 5723,756; 5,593,881; Geiser, et al.,(1986) Gene 48:109); lectins (Van Damme, et al., (1994) Plant Mol. Biol.24:825); fumonisin detoxification genes (U.S. Pat. No. 5,792,931);avirulence and disease resistance genes (Jones, et al., (1994) Science266:789; Martin, et al., (1993) Science 262:1432; Mindrinos, et al.,(1994) Cell 78:1089); acetolactate synthase (ALS) mutants that lead toherbicide resistance such as the S4 and/or Hra mutations; inhibitors ofglutamine synthase such as phosphinothricin or basta (e.g., bar gene)and glyphosate resistance (EPSPS gene)) and traits desirable forprocessing or process products such as high oil (e.g., U.S. Pat. No.6,232,529); modified oils (e.g., fatty acid desaturase genes (U.S. Pat.No. 5,952,544; WO 1994/11516)); modified starches (e.g., ADPGpyrophosphorylases (AGPase), starch synthases (SS), starch branchingenzymes (SBE) and starch debranching enzymes (SDBE)) and polymers orbioplastics (e.g., U.S. Pat. No. 5,602,321; beta-ketothiolase,polyhydroxybutyrate synthase and acetoacetyl-CoA reductase (Schubert, etal., (1988) J. Bacteriol. 170:5837-5847) facilitate expression ofpolyhydroxyalkanoates (PHAs)), the disclosures of which are hereinincorporated by reference. One could also combine the polynucleotides ofthe present invention with polynucleotides affecting agronomic traitssuch as male sterility (e.g., see U.S. Pat. No. 5,583,210), stalkstrength, flowering time or transformation technology traits such ascell cycle regulation or gene targeting (e.g., WO 1999/61619; WO2000/17364; WO 1999/25821), the disclosures of which are hereinincorporated by reference.

In one embodiment, sequences of interest improve plant growth and/orcrop yields. For example, sequences of interest include agronomicallyimportant genes that result in improved primary or lateral root systems.Such genes include, but are not limited to, nutrient/water transportersand growth induces. Examples of such genes, include but are not limitedto, maize plasma membrane H⁺-ATPase (MHA2) (Frias, et al., (1996) PlantCell 8:1533-44); AKT1, a component of the potassium uptake apparatus inArabidopsis, (Spalding, et al., (1999) J Gen Physiol 113:909-18); RMLgenes which activate cell division cycle in the root apical cells(Cheng, et al., (1995) Plant Physiol 108:881); maize glutaminesynthetase genes (Sukanya, et al., (1994) Plant Mol Biol 26:1935-46) andhemoglobin (Duff, et al., (1997) J. Biol. Chem. 27:16749-16752,Arredondo-Peter, et al., (1997) Plant Physiol. 115:1259-1266;Arredondo-Peter, et al., (1997) Plant Physiol 114:493-500 and referencessited therein). The sequence of interest may also be useful inexpressing antisense nucleotide sequences of genes that that negativelyaffects root development.

Additional, agronomically important traits such as oil, starch andprotein content can be genetically altered in addition to usingtraditional breeding methods. Modifications include increasing contentof oleic acid, saturated and unsaturated oils, increasing levels oflysine and sulfur, providing essential amino acids and also modificationof starch. Hordothionin protein modifications are described in U.S. Pat.Nos. 5,703,049, 5,885,801, 5,885,802 and 5,990,389, herein incorporatedby reference. Another example is lysine and/or sulfur rich seed proteinencoded by the soybean 2S albumin described in U.S. Pat. No. 5,850,016and the chymotrypsin inhibitor from barley, described in Williamson, etal., (1987) Eur. J. Biochem. 165:99-106, the disclosures of which areherein incorporated by reference.

Derivatives of the coding sequences can be made by site-directedmutagenesis to increase the level of preselected amino acids in theencoded polypeptide. For example, the gene encoding the barley highlysine polypeptide (BHL) is derived from barley chymotrypsin inhibitor,U.S. patent application Ser. No. 08/740,682, filed Nov. 1, 1996 and WO1998/20133, the disclosures of which are herein incorporated byreference. Other proteins include methionine-rich plant proteins such asfrom sunflower seed (Lilley, et al., (1989) Proceedings of the WorldCongress on Vegetable Protein Utilization in Human Foods and AnimalFeedstuffs, ed. Applewhite (American Oil Chemists Society, Champaign,Ill.), pp. 497-502; herein incorporated by reference); corn (Pedersen,et al., (1986) J. Biol. Chem. 261:6279; Kirihara, et al., (1988) Gene71:359, both of which are herein incorporated by reference) and rice(Musumura, et al., (1989) Plant Mol. Biol. 12:123, herein incorporatedby reference). Other agronomically important genes encode latex, Floury2, growth factors, seed storage factors and transcription factors.

Insect resistance genes may encode resistance to pests that have greatyield drag such as rootworm, cutworm, European Corn Borer, and the like.Such genes include, for example, Bacillus thuringiensis toxic proteingenes (U.S. Pat. Nos. 5,366,892; 5,747,450; 5,736,514; 5,723,756;5,593,881 and Geiser, et al., (1986) Gene 48:109); and the like.

Genes encoding disease resistance traits include detoxification genes,such as against fumonosin (U.S. Pat. No. 5,792,931); avirulence (avr)and disease resistance (R) genes (Jones, et al., (1994) Science 266:789;Martin, et al., (1993) Science 262:1432 and Mindrinos, et al., (1994)Cell 78:1089), and the like.

Herbicide resistance traits may include genes coding for resistance toherbicides that act to inhibit the action of acetolactate synthase(ALS), in particular the sulfonylurea-type herbicides (e.g., theacetolactate synthase (ALS) gene containing mutations leading to suchresistance, in particular the S4 and/or Hra mutations), genes coding forresistance to herbicides that act to inhibit action of glutaminesynthase, such as phosphinothricin or basta (e.g., the bar gene) orother such genes known in the art. The bar gene encodes resistance tothe herbicide basta, the nptII gene encodes resistance to theantibiotics kanamycin and geneticin and the ALS-gene mutants encoderesistance to the herbicide chlorsulfuron.

Sterility genes can also be encoded in an expression cassette andprovide an alternative to physical detasseling. Examples of genes usedin such ways include male tissue-preferred genes and genes with malesterility phenotypes such as QM, described in U.S. Pat. No. 5,583,210.Other genes include kinases and those encoding compounds toxic to eithermale or female gametophytic development.

The quality of grain is reflected in traits such as levels and types ofoils, saturated and unsaturated, quality and quantity of essential aminoacids and levels of cellulose. In corn, modified hordothionin proteinsare described in U.S. Pat. Nos. 5,703,049, 5,885,801, 5,885,802 and5,990,389.

Commercial traits can also be encoded on a gene or genes that couldincrease for example, starch for ethanol production or provideexpression of proteins. Another important commercial use of transformedplants is the production of polymers and bioplastics such as describedin U.S. Pat. No. 5,602,321. Genes such as 13-Ketothiolase, PHBase(polyhydroxyburyrate synthase) and acetoacetyl-CoA reductase (see,Schubert, et al., (1988) J. Bacteriol. 170:5837-5847) facilitateexpression of polyhyroxyalkanoates (PHAs).

Exogenous products include plant enzymes and products as well as thosefrom other sources including procaryotes and other eukaryotes. Suchproducts include enzymes, cofactors, hormones, and the like. The levelof proteins, particularly modified proteins having improved amino aciddistribution to improve the nutrient value of the plant, can beincreased. This is achieved by the expression of such proteins havingenhanced amino acid content.

This invention can be better understood by reference to the followingnon-limiting examples. It will be appreciated by those skilled in theart that other embodiments of the invention may be practiced withoutdeparting from the spirit and the scope of the invention as hereindisclosed and claimed.

EXAMPLES Example 1 Isolation of NR Sequences

20,778 Porphyra yezoensis ESTs were identified in Genbank thendownloaded. The dataset was converted to fasta, formatted for BLASTalgorithm searching and searched with maize nitrate reductase as thequery.

Multiple ESTs likely encoding fragments of nitrate reductase genes wereidentified, and EST contigs were built, supplemented by 5′- and3′-directional EST walking and final contig assembly of the genetranscript region. Prior to this contig assembly the gene and its codingregion existed in the public databases as short EST sequencesrepresenting merely fragments of the gene. The resulting Porphyrayezoensis nitrate reductase transcript contig represented a 5′UTR, theN-terminus and all but the last (C-terminal) 167 codons of the peptide.

Assuming the two strains of Porphyra were closely related organisms,oligonucleotide primers were designed from the assembled Porphyrayezoensis nitrate reductase gene with sense: attgtggtgcacaacaaggtgtatga(SEQ ID NO: 13) and anti-sense primers: tcggcttccacatgttcctg (SEQ ID NO:14). These primers were used to amplify a 448 bp internal region of thegene using Porphyra perforata genomic DNA. A Genome Walking kit from BioS&T (Montreal, Quebec, Canada) was then used to determine both ends ofthe gene, walking out from the internal fragment. (Standard conditionsfor nested PCR, ligation and digestion as per the provided SOP from BioS&T were used). For the Porphyra perforata gene discovery, the followingwalking primers were designed—ccgtgtacctccgcaactct (SEQ ID NO: 15),gctgcactttgtgcgcaacc (SEQ ID NO: 16), gcatgggacgagggcaacaa (SEQ ID NO:17), gcttgcccgtcggcttccacatgtt (SEQ ID NO: 18), catcgaaggctccatggtcatgcg(SEQ ID NO: 19) and ctacacgccaacgtcgtctgacgca (SEQ ID NO: 20).

Again assuming the Porphyra were closely related organisms, Porphyraperforate primers for full length nitrate reductase were used to amplifythe gene from the closely related Porphyra yezoensis red algae, but witha lower annealing temperature of 52° C. in the initial PCR set-up for 40cycles. Regions were sequence verified as correct, then using the sameGenome Walking kit from Bio S&T ambiguous regions and the remaining endsof the Porphyra yezoensis along with associated untranslated regionswere determined.

The entire ORF of the nitrate reductase gene from both species wascloned as a large PCR product using a high-fidelity Taq polymerase and aminimal of at least three independent clones was used to verify thecorrect internal sequence. The ORF of the Porphyra perforate nitratereductase gene was amplified with sense: catggaggctgcttctggtgc (SEQ IDNO: 21) and anti-sense primers: tcaacgagctgcttgtgggca (SEQ ID NO: 22).The ORF of the Porphyra yezoensis nitrate reductase gene was amplifiedwith sense: cccatcaccgcacagccga (SEQ ID NO: 23) and anti-sense primers:agagaggcgccccttgcatgtt (SEQ ID NO: 24). For most regions of the gene,more clones were used to validate the full-length sequence. Protein andnucleotide alignments confirmed that both genes were closely associatedand had many conserved protein residues when compared to other nitratereductase enzymes. There are no introns for either Nitrate Reductasecoding DNA sequence.

Example 2 Pichia pastoris Expression Vectors and Yeast Transformation

Pichia pastoris is a non-nitrate assimilating yeast and requires bothfunctional nitrate transporter and nitrate reductase to uptake nitrate(Unkles, et al., (2004) J. Biol. Chem. 279:28182-28186). P. pastoris hasbeen used as a model system for identification and characterization ofnitrate reductases and/or plant nitrate transporters. The nitratereductase activity can be determined in vitro or in vivo when a nitratetransporter gene was co-expressed. The wild type nitrate reductase genesfrom Porphyra performa (PpNR) and Porphyra yezoensis (PyNR) driven by ayeast constitutive promoter (GAP promoter, glyceraldehydes-3-phosphatedehydrogenase) was integrated into AOX1 locus of P. pastoris stain KM71carrying a yeast nitrate transporter gene (YNT1).

Details: YNT1 (nitrate transporter from Pichia angusta) driven by GAPpromoter (vector: p3.5GAP-YNT1) was integrated into P. pastoris strainKM71 genome at His4 locus to generate YNT1-containing lines. After theYNT1-containing lines were confirmed by PCR, PpNR or PyNR driven by GAPpromoter was integrated into the genome at AOX1 locus (vectors:pAOXGAP-PPNR and PAOXGAP-PYNR). (Pichia transformation kit is fromInvitrogen. Both expression vector backbones were modified based on theversions from Invitrogen.)

Functional Expression in P. pastoris:

Recombinant KM71 transformed with p3.5GAP-YNT1 and/or pAOXGAP-PPNR,pAOXGAP-PYNR were screened for NR activity. Transformants were culturedin rich media (YPD) at 30° C. for overnight. Yeast cells were collectedand washed with water twice then re-suspended in 20 uM MOPS, pH6.5 and1% glucose containing 1 mM NaNO₃. After 2 hours incubation at 30° C.,the supernatant was collected for nitrite assay with 2% and 2%. The Vmaxof PpNR is 70-80× higher than Py-NR. The transformants containing PpNRshowed much stronger nitrate reductase activity than PyNR in P. pastorisin vivo.

Kinetics in P. pastoris:

The nitrite concentration of P. pastoris KM71-containing YNT1 and PpNRline incubated with different nitrate concentration from 0-30 mM atthree pH, pH5.5, 6.5 and 7.5 was assayed in vivo. PpNR has better Vmaxat pH6.5. The Km is about 30-40 μM. This data match the predictedresults (published PyNR with Km 64 μM). At the same time, maize NR,ZM-NR and yeast NR from P. angusta, YNR1, were assayed at similarconditions. The Km of ZM-NR is about 300 uM and YNR1 has Km about200-400 μM. They also have the better Vmax at pH6.5. The PPNR is anefficient NR with Km 10× lower than others.

Example 3 PpNR and PyNR Mutants

To maintain high level of active form of PPNR in transgenic plants, theserine residue in a putative phosphorylation motif R/K-S/T-X-pS-X-P (J.of Experimental Botany (2004) 55:1275-1282) at position 561 will bemutated to either alanine (S561A mutant, sequence provided) or asparticacid (S561D mutant). The mutated PPNRs will be generated in P. pastorisand tested in YNT1-carrying line for NR activity as described before.The functional S561A or S561D will be expressed in transgenic plantsdriven by a constitutive promoter or a mesophyll preferred promoter. TheNR activity will be tested during light/dark transitions. Other putativepost-translational regions such as N-terminal region of PPNR may bemodified or deleted to de-regulate by light at post-translational level.

Example 4 Transformation and Regeneration of Transgenic Plants

Immature maize embryos from greenhouse donor plants are bombarded with aplasmid containing the NR sequence operably linked to thedrought-inducible promoter RAB17 promoter (Vilardell, et al., (1990)Plant Mol Biol 14:423-432) and the selectable marker gene PAT, whichconfers resistance to the herbicide Bialaphos. (Miller, et al., (2000)Biochimica et Biophysica Acta 1465:343-358). Alternatively, theselectable marker gene is provided on a separate plasmid. Transformationis performed as follows. Media recipes follow below.

Preparation of Target Tissue:

The ears are husked and surface sterilized in 30% Clorox® bleach plus0.5% Micro detergent for 20 minutes, and rinsed two times with sterilewater. The immature embryos are excised and placed embryo axis side down(scutellum side up), 25 embryos per plate, on 560Y medium for 4 hoursand then aligned within the 2.5-cm target zone in preparation forbombardment.

Preparation of DNA:

A plasmid vector comprising the NR sequence operably linked to anubiquitin promoter is made. This plasmid DNA plus plasmid DNA containinga PAT selectable marker is precipitated onto 1.1 μm (average diameter)tungsten pellets using a CaCl₂ precipitation procedure as follows:

100 μl prepared tungsten particles in water10 μl (1 μg) DNA in Tris EDTA buffer (1 μg total DNA)

100 μl 2.5 M CaCl₂

10 μl 0.1 M spermidine

Each reagent is added sequentially to the tungsten particle suspension,while maintained on the multitube vortexer. The final mixture issonicated briefly and allowed to incubate under constant vortexing for10 minutes. After the precipitation period, the tubes are centrifugedbriefly, liquid removed, washed with 500 ml 100% ethanol and centrifugedfor 30 seconds. Again the liquid is removed, and 105 μl 100% ethanol isadded to the final tungsten particle pellet. For particle gunbombardment, the tungsten/DNA particles are briefly sonicated and 10 μlspotted onto the center of each macrocarrier and allowed to dry about 2minutes before bombardment.

Particle Gun Treatment:

The sample plates are bombarded at level #4 in a particle gun. Allsamples receive a single shot at 650 PSI, with a total of ten aliquotstaken from each tube of prepared particles/DNA.

Subsequent Treatment:

Following bombardment, the embryos are kept on 560Y medium for 2 days,then transferred to 560R selection medium containing 3 mg/literBialaphos, and subcultured every 2 weeks. After approximately 10 weeksof selection, selection-resistant callus clones are transferred to 288Jmedium to initiate plant regeneration. Following somatic embryomaturation (2-4 weeks), well-developed somatic embryos are transferredto medium for germination and transferred to the lighted culture room.Approximately 7-10 days later, developing plantlets are transferred to272V hormone-free medium in tubes for 7-10 days until plantlets are wellestablished. Plants are then transferred to inserts in flats (equivalentto 2.5″ pot) containing potting soil and grown for 1 week in a growthchamber, subsequently grown an additional 1-2 weeks in the greenhouse,then transferred to classic 600 pots (1.6 gallon) and grown to maturity.Plants are monitored and scored for increased drought tolerance. Assaysto measure improved drought tolerance are routine in the art andinclude, for example, increased kernel-earring capacity yields underdrought conditions when compared to control maize plants under identicalenvironmental conditions. Alternatively, the transformed plants can bemonitored for a modulation in meristem development (i.e., a decrease inspikelet formation on the ear). See, for example, Bruce, et al., (2002)Journal of Experimental Botany 53:1-13.

Bombardment and Culture Media:

Bombardment medium (560Y) comprises 4.0 g/l N6 basal salts (SIGMAC-1416), 1.0 ml/l Eriksson's Vitamin Mix (1000×SIGMA-1511), 0.5 mg/lthiamine HCl, 120.0 g/l sucrose, 1.0 mg/l 2,4-D and 2.88 g/l L-proline(brought to volume with D-I H₂O following adjustment to pH 5.8 withKOH); 2.0 g/l Gelrite® (added after bringing to volume with D-I H₂O);and 8.5 mg/l silver nitrate (added after sterilizing the medium andcooling to room temperature). Selection medium (560R) comprises 4.0 g/lN6 basal salts (SIGMA C-1416), 1.0 ml/I Eriksson's Vitamin Mix(1000×SIGMA-1511), 0.5 mg/l thiamine HCl, 30.0 g/l sucrose, and 2.0 mg/l2,4-D (brought to volume with D-I H₂O following adjustment to pH 5.8with KOH); 3.0 g/l Gelrite® (added after bringing to volume with D-IH₂O) and 0.85 mg/l silver nitrate and 3.0 mg/l bialaphos (both addedafter sterilizing the medium and cooling to room temperature).

Plant regeneration medium (288J) comprises 4.3 g/l MS salts (GIBCO11117-074), 5.0 ml/l MS vitamins stock solution (0.100 g nicotinic acid,0.02 g/l thiamine HCL, 0.10 g/l pyridoxine HCL and 0.40 g/l glycinebrought to volume with polished D-I H₂O) (Murashige and Skoog, (1962)Physiol. Plant. 15:473), 100 mg/l myo-inositol, 0.5 mg/l zeatin, 60 g/lsucrose and 1.0 ml/I of 0.1 mM abscisic acid (brought to volume withpolished D-I H₂O after adjusting to pH 5.6); 3.0 g/l Gelrite® (addedafter bringing to volume with D-I H₂O) and 1.0 mg/l indoleacetic acidand 3.0 mg/l bialaphos (added after sterilizing the medium and coolingto 60° C.). Hormone-free medium (272V) comprises 4.3 g/l MS salts (GIBCO11117-074), 5.0 ml/I MS vitamins stock solution (0.100 g/l nicotinicacid, 0.02 g/l thiamine HCL, 0.10 g/l pyridoxine HCL and 0.40 g/lglycine brought to volume with polished D-I H₂O), 0.1 g/1 myo-inositoland 40.0 g/l sucrose (brought to volume with polished D-I H₂O afteradjusting pH to 5.6) and 6 g/l Bacto™-agar (added after bringing tovolume with polished D-I H₂O), sterilized and cooled to 60° C.

Example 5 Agrobacterium-Mediated Transformation

For Agrobacterium-mediated transformation of maize with an antisensesequence of the NR sequence of the present invention, preferably themethod of Zhao is employed (U.S. Pat. No. 5,981,840 and PCT PatentApplication Publication WO 1998/32326, the contents of which are herebyincorporated by reference). Briefly, immature embryos are isolated frommaize and the embryos contacted with a suspension of Agrobacterium,where the bacteria are capable of transferring the antisense NRsequences to at least one cell of at least one of the immature embryos(step 1: the infection step). In this step the immature embryos arepreferably immersed in an Agrobacterium suspension for the initiation ofinoculation. The embryos are co-cultured for a time with theAgrobacterium (step 2: the co-cultivation step). Preferably the immatureembryos are cultured on solid medium following the infection step.Following this co-cultivation period an optional “resting” step iscontemplated. In this resting step, the embryos are incubated in thepresence of at least one antibiotic known to inhibit the growth ofAgrobacterium without the addition of a selective agent for planttransformants (step 3: resting step). Preferably the immature embryosare cultured on solid medium with antibiotic, but without a selectingagent, for elimination of Agrobacterium and for a resting phase for theinfected cells. Next, inoculated embryos are cultured on mediumcontaining a selective agent and growing transformed callus is recovered(step 4: the selection step). Preferably, the immature embryos arecultured on solid medium with a selective agent resulting in theselective growth of transformed cells. The callus is then regeneratedinto plants (step 5: the regeneration step) and preferably calli grownon selective medium are cultured on solid medium to regenerate theplants. Plants are monitored and scored for a modulation in meristemdevelopment (for instance, alterations of size and appearance of theshoot and floral meristems and/or increased yields of leaves, flowersand/or fruits).

Example 6 Soybean Embryo Transformation

Soybean embryos are bombarded with a plasmid containing an antisense NRsequences operably linked to an ubiquitin promoter as follows. To inducesomatic embryos, cotyledons, 3-5 mm in length dissected fromsurface-sterilized, immature seeds of the soybean cultivar A2872, arecultured in the light or dark at 26° C. on an appropriate agar mediumfor six to ten weeks. Somatic embryos producing secondary embryos arethen excised and placed into a suitable liquid medium. After repeatedselection for clusters of somatic embryos that multiplied as early,globular-staged embryos, the suspensions are maintained as describedbelow.

Soybean embryogenic suspension cultures can be maintained in 35 mlliquid media on a rotary shaker, 150 rpm, at 26° C. with florescentlights on a 16:8 hour day/night schedule. Cultures are subcultured everytwo weeks by inoculating approximately 35 mg of tissue into 35 ml ofliquid medium.

Soybean embryogenic suspension cultures may then be transformed by themethod of particle gun bombardment (Klein, et al., (1987) Nature(London) 327:70-73, U.S. Pat. No. 4,945,050). A Du Pont BiolisticPDS1000/HE instrument (helium retrofit) can be used for thesetransformations.

A selectable marker gene that can be used to facilitate soybeantransformation is a transgene composed of the ³⁵S promoter fromCauliflower Mosaic Virus (Odell, et al., (1985) Nature 313:810-812), thehygromycin phosphotransferase gene from plasmid pJR225 (from E. coli;Gritz, et al., (1983) Gene 25:179-188) and the 3′ region of the nopalinesynthase gene from the T-DNA of the Ti plasmid of Agrobacteriumtumefaciens. The expression cassette comprising an antisense NR sequenceoperably linked to the ubiquitin promoter can be isolated as arestriction fragment. This fragment can then be inserted into a uniquerestriction site of the vector carrying the marker gene.

To 50 μl of a 60 mg/ml 1 μm gold particle suspension is added (inorder): 5 μl DNA (1 μg/μl), 20 μl spermidine (0.1 M) and 50 μl CaCl₂(2.5 M). The particle preparation is then agitated for three minutes,spun in a microfuge for 10 seconds and the supernatant removed. TheDNA-coated particles are then washed once in 400 μl 70% ethanol andresuspended in 40 μl of anhydrous ethanol. The DNA/particle suspensioncan be sonicated three times for one second each. Five microliters ofthe DNA-coated gold particles are then loaded on each macro carrierdisk.

Approximately 300-400 mg of a two-week-old suspension culture is placedin an empty 60×15 mm petri dish and the residual liquid removed from thetissue with a pipette. For each transformation experiment, approximately5-10 plates of tissue are normally bombarded. Membrane rupture pressureis set at 1100 psi, and the chamber is evacuated to a vacuum of 28inches mercury. The tissue is placed approximately 3.5 inches away fromthe retaining screen and bombarded three times. Following bombardment,the tissue can be divided in half and placed back into liquid andcultured as described above.

Five to seven days post bombardment, the liquid media may be exchangedwith fresh media, and eleven to twelve days post-bombardment with freshmedia containing 50 mg/ml hygromycin. This selective media can berefreshed weekly. Seven to eight weeks post-bombardment, green,transformed tissue may be observed growing from untransformed, necroticembryogenic clusters. Isolated green tissue is removed and inoculatedinto individual flasks to generate new, clonally propagated, transformedembryogenic suspension cultures. Each new line may be treated as anindependent transformation event. These suspensions can then besubcultured and maintained as clusters of immature embryos orregenerated into whole plants by maturation and germination ofindividual somatic embryos.

Example 7 Sunflower Meristem Tissue Transformation

Sunflower meristem tissues are transformed with an expression cassettecontaining an antisense NR sequences operably linked to a ubiquitinpromoter as follows (see also, European Patent Number EP 0 486233,herein incorporated by reference and Malone-Schoneberg, et al., (1994)Plant Science 103:199-207). Mature sunflower seed (Helianthus annuus L.)are dehulled using a single wheat-head thresher. Seeds are surfacesterilized for 30 minutes in a 20% Clorox® bleach solution with theaddition of two drops of Tween® 20 per 50 ml of solution. The seeds arerinsed twice with sterile distilled water.

Split embryonic axis explants are prepared by a modification ofprocedures described by Schrammeijer, et al. (Schrammeijer, et al.,(1990) Plant Cell Rep. 9:55-60). Seeds are imbibed in distilled waterfor 60 minutes following the surface sterilization procedure. Thecotyledons of each seed are then broken off, producing a clean fractureat the plane of the embryonic axis. Following excision of the root tip,the explants are bisected longitudinally between the primordial leaves.The two halves are placed, cut surface up, on GBA medium consisting ofMurashige and Skoog mineral elements (Murashige, et al., (1962) Physiol.Plant., 15:473-497), Shepard's vitamin additions (Shepard, (1980) inEmergent Techniques for the Genetic Improvement of Crops (University ofMinnesota Press, St. Paul, Minn.), 40 mg/l adenine sulfate, 30 g/lsucrose, 0.5 mg/l 6-benzyl-aminopurine (BAP), 0.25 mg/l indole-3-aceticacid (IAA), 0.1 mg/l gibberellic acid (GA₃), pH 5.6, and 8 g/l Phytagar.

The explants are subjected to microprojectile bombardment prior toAgrobacterium treatment (Bidney, et al., (1992) Plant Mol. Biol.18:301-313). Thirty to forty explants are placed in a circle at thecenter of a 60×20 mm plate for this treatment. Approximately 4.7 mg of1.8 mm tungsten microprojectiles are resuspended in 25 ml of sterile TEbuffer (10 mM Tris HCl, 1 mM EDTA, pH 8.0) and 1.5 ml aliquots are usedper bombardment. Each plate is bombarded twice through a 150 mm nytexscreen placed 2 cm above the samples in a PDS 1000® particleacceleration device.

Disarmed Agrobacterium tumefaciens strain EHA105 is used in alltransformation experiments. A binary plasmid vector comprising theexpression cassette that contains the NR gene operably linked to theubiquitin promoter is introduced into Agrobacterium strain EHA105 viafreeze-thawing as described by Holsters, et al., (1978) Mol. Gen. Genet.163:181-187. This plasmid further comprises a kanamycin selectablemarker gene (i.e, nptII). Bacteria for plant transformation experimentsare grown overnight (28° C. and 100 RPM continuous agitation) in liquidYEP medium (10 gm/l yeast extract, 10 gm/l Bacto®peptone and 5 gm/lNaCl, pH 7.0) with the appropriate antibiotics required for bacterialstrain and binary plasmid maintenance. The suspension is used when itreaches an OD₆₀₀ of about 0.4 to 0.8. The Agrobacterium cells arepelleted and resuspended at a final OD₆₀₀ of 0.5 in an inoculationmedium comprised of 12.5 mM MES pH 5.7, 1 gm/l NH₄Cl and 0.3 gm/l MgSO₄.

Freshly bombarded explants are placed in an Agrobacterium suspension,mixed and left undisturbed for 30 minutes. The explants are thentransferred to GBA medium and co-cultivated, cut surface down, at 26° C.and 18-hour days. After three days of co-cultivation, the explants aretransferred to 374B (GBA medium lacking growth regulators and a reducedsucrose level of 1%) supplemented with 250 mg/l cefotaxime and 50 mg/lkanamycin sulfate. The explants are cultured for two to five weeks onselection and then transferred to fresh 374B medium lacking kanamycinfor one to two weeks of contiNRd development. Explants withdifferentiating, antibiotic-resistant areas of growth that have notproduced shoots suitable for excision are transferred to GBA mediumcontaining 250 mg/l cefotaxime for a second 3-day phytohormonetreatment. Leaf samples from green, kanamycin-resistant shoots areassayed for the presence of NPTII by ELISA and for the presence oftransgene expression by assaying for a modulation in meristemdevelopment (i.e., an alteration of size and appearance of shoot andfloral meristems).

NPTII-positive shoots are grafted to Pioneer® hybrid 6440 in vitro-grownsunflower seedling rootstock. Surface sterilized seeds are germinated in48-0 medium (half-strength Murashige and Skoog salts, 0.5% sucrose, 0.3%Gelrite®, pH 5.6) and grown under conditions described for explantculture. The upper portion of the seedling is removed, a 1 cm verticalslice is made in the hypocotyl and the transformed shoot inserted intothe cut. The entire area is wrapped with Parafilm® to secure the shoot.Grafted plants can be transferred to soil following one week of in vitroculture. Grafts in soil are maintained under high humidity conditionsfollowed by a slow acclimatization to the greenhouse environment.Transformed sectors of T₀ plants (parental generation) maturing in thegreenhouse are identified by NPTII ELISA and/or by NR activity analysisof leaf extracts while transgenic seeds harvested from NPTII-positive T₀plants are identified by NR activity analysis of small portions of dryseed cotyledon.

An alternative sunflower transformation protocol allows the recovery oftransgenic progeny without the use of chemical selection pressure. Seedsare dehulled and surface-sterilized for 20 minutes in a 20% Clorox®bleach solution with the addition of two to three drops of Tween® 20 per100 ml of solution, then rinsed three times with distilled water.Sterilized seeds are imbibed in the dark at 26° C. for 20 hours onfilter paper moistened with water. The cotyledons and root radical areremoved, and the meristem explants are cultured on 374E (GBA mediumconsisting of MS salts, Shepard vitamins, 40 mg/l adenine sulfate, 3%sucrose, 0.5 mg/l 6-BAP, 0.25 mg/l IAA, 0.1 mg/l GA, and 0.8% Phytagarat pH 5.6) for 24 hours under the dark. The primary leaves are removedto expose the apical meristem, around 40 explants are placed with theapical dome facing upward in a 2 cm circle in the center of 374M (GBAmedium with 1.2% Phytagar) and then cultured on the medium for 24 hoursin the dark.

Approximately 18.8 mg of 1.8 μm tungsten particles are resuspended in150 μl absolute ethanol. After sonication, 8 μl of it is dropped on thecenter of the surface of macrocarrier. Each plate is bombarded twicewith 650 psi rupture discs in the first shelf at 26 mm of Hg helium gunvacuum.

The plasmid of interest is introduced into Agrobacterium tumefaciensstrain EHA105 via freeze thawing as described previously. The pellet ofovernight-grown bacteria at 28° C. in a liquid YEP medium (10 g/l yeastextract, 10 g/l Bacto®peptone and 5 g/l NaCl, pH 7.0) in the presence of50 μg/l kanamycin is resuspended in an inoculation medium (12.5 mM 2-mM2-(N-morpholino) ethanesulfonic acid, MES, 1 g/l NH₄Cl and 0.3 g/l MgSO₄at pH 5.7) to reach a final concentration of 4.0 at OD₆₀₀.Particle-bombarded explants are transferred to GBA medium (374E) and adroplet of bacteria suspension is placed directly onto the top of themeristem. The explants are co-cultivated on the medium for 4 days, afterwhich the explants are transferred to 374C medium (GBA with 1% sucroseand no BAP, IAA, GA3 and supplemented with 250 μg/ml cefotaxime). Theplantlets are cultured on the medium for about two weeks under 16-hourday and 26° C. incubation conditions.

Explants (around 2 cm long) from two weeks of culture in 374C medium arescreened for a modulation in meristem development (i.e., an alterationof size and appearance of shoot and floral meristems). After positiveexplants are identified, those shoots that fail to exhibit modified NRactivity are discarded, and every positive explant is subdivided intonodal explants. One nodal explant contains at least one potential node.The nodal segments are cultured on GBA medium for three to four days topromote the formation of auxiliary buds from each node. Then they aretransferred to 374C medium and allowed to develop for an additional fourweeks. Developing buds are separated and cultured for an additional fourweeks on 374C medium. Pooled leaf samples from each newly recoveredshoot are screened again by the appropriate protein activity assay. Atthis time, the positive shoots recovered from a single node willgenerally have been enriched in the transgenic sector detected in theinitial assay prior to nodal culture.

Recovered shoots positive for modified NR expression are grafted toPioneer hybrid 6440 in vitro-grown sunflower seedling rootstock. Therootstocks are prepared in the following manner. Seeds are dehulled andsurface-sterilized for 20 minutes in a 20% Clorox® bleach solution withthe addition of two to three drops of Tween® 20 per 100 ml of solution,and are rinsed three times with distilled water. The sterilized seedsare germinated on the filter moistened with water for three days, thenthey are transferred into 48 medium (half-strength MS salt, 0.5%sucrose, 0.3% Gelrite® pH 5.0) and grown at 26° C. under the dark forthree days, then incubated at 16-hour-day culture conditions. The upperportion of selected seedling is removed, a vertical slice is made ineach hypocotyl and a transformed shoot is inserted into a V-cut. The cutarea is wrapped with Parafilm®. After one week of culture on the medium,grafted plants are transferred to soil. In the first two weeks, they aremaintained under high humidity conditions to acclimatize to a greenhouseenvironment.

Example 8 Rice Tissue Transformation Genetic Confirmation of the NR Gene

One method for transforming DNA into cells of higher plants that isavailable to those skilled in the art is high-velocity ballisticbombardment using metal particles coated with the nucleic acidconstructs of interest (see, Klein, et al., Nature (1987) (London)327:70-73 and see, U.S. Pat. No. 4,945,050). A Biolistic PDS-1000/He(BioRAD Laboratories, Hercules, Calif.) is used for thesecomplementation experiments. The particle bombardment technique is usedto transform the NR mutants and wild type rice with DNA fragments

The bacterial hygromycin B phosphotransferase (Hpt II) gene fromStreptomyces hygroscopicus that confers resistance to the antibiotic isused as the selectable marker for rice transformation. In the vector,pML18, the Hpt II gene was engineered with the ³⁵S promoter fromCauliflower Mosaic Virus and the termination and polyadenylation signalsfrom the octopine synthase gene of Agrobacterium tumefaciens. pML18 wasdescribed in WO 1997/47731, which was published on Dec. 18, 1997, thedisclosure of which is hereby incorporated by reference.

Embryogenic callus cultures derived from the scutellum of germinatingrice seeds serve as source material for transformation experiments. Thismaterial is generated by germinating sterile rice seeds on a callusinitiation media (MS salts, Nitsch and Nitsch vitamins, 1.0 mg/l 2,4-Dand 10 μM AgNO₃) in the dark at 27-28° C. Embryogenic callusproliferating from the scutellum of the embryos is the transferred to CMmedia (N6 salts, Nitsch and Nitsch vitamins, 1 mg/l 1 2,4-D, Chu, etal., (1985) Sci. Sinica 18: 659-668). Callus cultures are maintained onCM by routine sub-culture at two week intervals and used fortransformation within 10 weeks of initiation.

Callus is prepared for transformation by subculturing 0.5-1.0 mm piecesapproximately 1 mm apart, arranged in a circular area of about 4 cm indiameter, in the center of a circle of Whatman® #541 paper placed on CMmedia. The plates with callus are incubated in the dark at 27-28° C. for3-5 days. Prior to bombardment, the filters with callus are transferredto CM supplemented with 0.25 M mannitol and 0.25 M sorbitol for 3 hr inthe dark. The petri dish lids are then left ajar for 20-45 minutes in asterile hood to allow moisture on tissue to dissipate.

Each genomic DNA fragment is co-precipitated with pML18 containing theselectable marker for rice transformation onto the surface of goldparticles. To accomplish this, a total of 10 μg of DNA at a 2:1 ratio oftrait:selectable marker DNAs are added to 50 μl aliquot of goldparticles that have been resuspended at a concentration of 60 mg ml⁻¹.Calcium chloride (50 μl of a 2.5 M solution) and spermidine (20 μl of a0.1 M solution) are then added to the gold-DNA suspension as the tube isvortexing for 3 min. The gold particles are centrifuged in a microfugefor 1 sec and the supernatant removed. The gold particles are thenwashed twice with 1 ml of absolute ethanol and then resuspended in 50 μlof absolute ethanol and sonicated (bath sonicator) for one second todisperse the gold particles. The gold suspension is incubated at −70° C.for five minutes and sonicated (bath sonicator) if needed to dispersethe particles. Six μl of the DNA-coated gold particles are then loadedonto mylar macrocarrier disks and the ethanol is allowed to evaporate.

At the end of the drying period, a petri dish containing the tissue isplaced in the chamber of the PDS-1000/He. The air in the chamber is thenevacuated to a vacuum of 28-29 inches Hg. The macrocarrier isaccelerated with a helium shock wave using a rupture membrane thatbursts when the He pressure in the shock tube reaches 1080-1100 psi. Thetissue is placed approximately 8 cm from the stopping screen and thecallus is bombarded two times. Two to four plates of tissue arebombarded in this way with the DNA-coated gold particles. Followingbombardment, the callus tissue is transferred to CM media withoutsupplemental sorbitol or mannitol.

Within 3-5 days after bombardment the callus tissue is transferred to SMmedia (CM medium containing 50 mg/l hygromycin). To accomplish this,callus tissue is transferred from plates to sterile 50 ml conical tubesand weighed. Molten top-agar at 40° C. is added using 2.5 ml of topagar/100 mg of callus. Callus clumps are broken into fragments of lessthan 2 mm diameter by repeated dispensing through a 10 ml pipet. Threeml aliquots of the callus suspension are plated onto fresh SM media andthe plates are incubated in the dark for 4 weeks at 27-28° C. After 4weeks, transgenic callus events are identified, transferred to fresh SMplates and grown for an additional 2 weeks in the dark at 27-28° C.

Growing callus is transferred to RM1 media (MS salts, Nitsch and Nitschvitamins, 2% sucrose, 3% sorbitol, 0.4% Gelrite®+50 ppm hyg B) for 2weeks in the dark at 25° C. After 2 weeks the callus is transferred toRM2 media (MS salts, Nitsch and Nitsch vitamins, 3% sucrose, 0.4%Gelrite®+50 ppm hyg B) and placed under cool white light (˜40 μEm⁻² s⁻¹)with a 12 hr photoperiod at 25° C. and 30-40% humidity. After 2-4 weeksin the light, callus begin to organize and form shoots. Shoots areremoved from surrounding callus/media and gently transferred to RM3media (½×MS salts, Nitsch and Nitsch vitamins, 1% sucrose+50 ppmhygromycin B) in Phytatrays™ (Sigma Chemical Co., St. Louis, Mo.) andincubation is contiNRd using the same conditions as described in theprevious step.

Plants are transferred from RM3 to 4″ pots containing Metro Mix® 350after 2-3 weeks, when sufficient root and shoot growth have occurred.The seed obtained from the transgenic plants is examined for geneticcomplementation of the NR mutation with the wild-type genomic DNAcontaining the NR gene.

Example 9 Variants of NR Sequences

A. Variant Nucleotide Sequences of NR that do not Alter the EncodedAmino Acid Sequence

The NR nucleotide sequences are used to generate variant nucleotidesequences having the nucleotide sequence of the open reading frame withabout 70%, 75%, 80%, 85%, 90% and 95% nucleotide sequence identity whencompared to the starting unaltered ORF nucleotide sequence of thecorresponding SEQ ID NO. These functional variants are generated using astandard codon table. While the nucleotide sequences of the variants arealtered, the amino acid sequences encoded by the open reading frames donot change.

B. Variant Amino Acid Sequences of NR Polypeptides

Variant amino acid sequences of the NR polypeptides are generated. Inthis example, one amino acid is altered. Specifically, the open readingframes are reviewed to determine the appropriate amino acid alteration.The selection of the amino acid to change is made by consulting theprotein alignment (with the other orthologs and other gene familymembers from various species). An amino acid is selected that is deemednot to be under high selection pressure (not highly conserved) and whichis rather easily substituted by an amino acid with similar chemicalcharacteristics (i.e., similar functional side-chain). Using the proteinalignment, an appropriate amino acid can be changed. Once the targetedamino acid is identified, the procedure outlined in the followingsection C is followed. Variants having about 70%, 75%, 80%, 85%, 90% and95% nucleic acid sequence identity are generated using this method.

C. Additional Variant Amino Acid Sequences of NR Polypeptides

In this example, artificial protein sequences are created having 80%,85%, 90% and 95% identity relative to the reference protein sequence.This latter effort requires identifying conserved and variable regionsfrom the alignment and then the judicious application of an amino acidsubstitutions table. These parts will be discussed in more detail below.

Largely, the determination of which amino acid sequences are altered ismade based on the conserved regions among NR protein or among the otherNR polypeptides. Based on the sequence alignment, the various regions ofthe NR polypeptide that can likely be altered are represented in lowercase letters, while the conserved regions are represented by capitalletters. It is recognized that conservative substitutions can be made inthe conserved regions below without altering function. In addition, oneof skill will understand that functional variants of the NR sequence ofthe invention can have minor non-conserved amino acid alterations in theconserved domain.

Artificial protein sequences are then created that are different fromthe original in the intervals of 80-85%, 85-90%, 90-95% and 95-100%identity. Midpoints of these intervals are targeted, with liberallatitude of plus or minus 1%, for example. The amino acids substitutionswill be effected by a custom Perl script. The substitution table isprovided below in Table 2.

TABLE 2 Substitution Table Rank of Amino Strongly Similar and Order toAcid Optimal Substitution Change Comment I L, V 1 50:50 substitution LI, V 2 50:50 substitution V I, L 3 50:50 substitution A G 4 G A 5 D E 6E D 7 W Y 8 Y W 9 S T 10 T S 11 K R 12 R K 13 N Q 14 Q N 15 F Y 16 M L17 First methionine cannot change H Na No good substitutes C Na No goodsubstitutes P Na No good substitutes

First, any conserved amino acids in the protein that should not bechanged is identified and “marked off” for insulation from thesubstitution. The start methionine will of course be added to this listautomatically. Next, the changes are made.

H, C and P are not changed in any circumstance. The changes will occurwith isoleucine first, sweeping N-terminal to C-terminal. Then leucine,and so on down the list until the desired target it reached. Interimnumber substitutions can be made so as not to cause reversal of changes.The list is ordered 1-17, so start with as many isoleucine changes asneeded before leucine, and so on down to methionine. Clearly many aminoacids will in this manner not need to be changed. L, I and V willinvolve a 50:50 substitution of the two alternate optimal substitutions.

The variant amino acid sequences are written as output. Perl script isused to calculate the percent identities. Using this procedure, variantsof the NR polypeptides are generating having about 80%, 85%, 90% and 95%amino acid identity to the starting ORF nucleotide sequence of SEQ IDNO: 1, 2, 3, 4, 5 or 6.

Example 10 Transgenic Maize Plants

T₀ transgenic maize plants containing the NR construct under the controlof a promoter were generated. To improve nitrate assimilation, PPNR andPYNR driven by constitutive promoter (ZM-UBI) and tissue-specificpromoter (ZM-NR) (4 constructs) were transformed in transgenic maizeplants via Agrobacteria.

These plants were grown in greenhouse conditions, under the FASTCORNsystem, as detailed in US Patent Application Publication Number2003/0221212, U.S. patent application Ser. No. 10/367,417.

Each of the plants was analyzed for measurable alteration in one or moreof the following characteristics in the following manner:

T₁ progeny derived from cross fertilization each T₀ plant containing asingle copy of each NR construct that were found to segregate 1:1 forthe transgenic event were analyzed for improved growth rate in low (1mM) KNO₃. Growth was monitored up to anthesis when cumulative plantgrowth, growth rate and ear weight were determined for transgenepositive, transgene null and non-transformed controls events. Thedistribution of the phenotype of individual plants was compared to thedistribution of a control set and to the distribution of all theremaining treatments. Transgenic means were compared to the grand mean,the block mean in which the transgene resides and the correspondingtransgenic null mean and tested for statistical significance using aStudents' t-test. Variances for each set were calculated and used as thedenominator of the Student's t-test after adjustment for the number ofobservations in each mean compared ie: Student's t test=(transgenemean−transgenic null mean)/Sqr(variance*(1/number of observations oftransgene mean+1/number of observations of transgene null mean)). Theprobability of obtaining the calculated Student's t-test at random wascalculated based on the magnitude of the Student's t-test and the numberof degrees of freedom associated with the variance. A probability of0.1(1 chance in 10 of obtaining the result at random) or lower was usedto indicate a significant response to KNO₃ fertility.

Example 11 Transgenic Event Analysis from Field Plots

Transgenic events are evaluated in field plots where yield is limited byreducing fertilizer application by 30% or more. Transgenic maize will begrown hydroponically. Improvements in yield, yield components or otheragronomic traits between transgenic and non-transgenic plants in thesereduced nitrogen fertility plots are used to assess improvements innitrogen utilization contributed by expression of transgenic events.

The rate of nitrate removal from hydroponics medium, the nitrate andtotal nitrogen level in the plant may be determined using routinetechniques. Nitrate levels in the growth medium will be monitored andcompared to the nitrate levels in the growth medium of transgenic nulls.

The rate of nitrate loss from the medium is an indication of nitrateutilization efficiency. Plant samples will be dried, ground and nitrateextracted and quantified. Total N will be determined in the tissue bymicro-Kjeldahl. (Yasuhura and Nokihara, (2001) J Agric Food Chem49:4581-4583). Comparisons are made in plots supplemented withrecommended nitrogen fertility rates. Effective transgenic events arethose that achieve similar yields in the nitrogen-limited and normalnitrogen experiments.

Example 12 Maize Backcross Analysis

Segregating T₄ backcrosses to Gaspe-3 were grown in nutrient solutionscontaining 1 or 2 mM KNO₃ as the sole nitrogen source till anthesis.Leaf color (SPAD), stem diameter, vegetative and ear dry weight weredetermined and compared to the corresponding segregating null plants.There were 9 replicates of all treatments. Results for the plantscontaining the PpNR transgene showed statistically significant (at 0.001level) improvement in ear dry weight (at both 1 and 2 mM KNO₃concentrations) in comparison to non-transgenic control plants. At 2 mMKNO₃, there were also statistically significant improvements noted inSPAD, ear dry weight and total dry weight, as compared to non-transgeniccontrol plants. This demonstrates the gene is stable after 5 cycles ofbackcrossing.

All publications and patent applications in this specification areindicative of the level of ordinary skill in the art to which thisinvention pertains. All publications and patent applications are hereinincorporated by reference to the same extent as if each individualpublication or patent application was specifically and individuallyindicated by reference.

The invention has been described with reference to various specific andpreferred embodiments and techniques. However, it should be understoodthat many variations and modifications may be made while remainingwithin the spirit and scope of the invention.

What is claimed is:
 1. An isolated nitrate reductase (NR) polynucleotidecomprising a member selected from the group consisting of: (a) apolynucleotide that encodes the polypeptide of SEQ ID NO: 7, 8, 9, 10,11 or 12; (b) a polynucleotide comprising the sequence set forth in SEQID NO: 1, 2, 3, 4, 5 or 6; and (c) a polynucleotide comprising at least30 nucleotides in length which hybridizes under stringent conditions toa polynucleotide of (a) or (b), wherein the conditions includehybridization in 40 to 45% formamide, 1 M NaCl, 1% SDS at 37° C. and awash in 0.5× to 1×SSC at 55 to 60° C.; and (d) a polynucleotide havingat least 70% sequence identity to SEQ ID NO: 1, 2, 3, 4, 5 or 6, whereinthe % sequence identity is based on the entire encoding region and isdetermined by BLAST 2.0 under default parameters wherein thepolynucleotide encodes a polypeptide having nitrate reductase (NR)activity; and (e) an isolated polynucleotide degenerate from any of (a)to (e) as a result of the genetic code; (f) a polynucleotidecomplimentary to a polynucleotide of any one of (a) to (e).
 2. Anisolated polynucleotide according to claim 1 that encodes a NRpolypeptide that confers increased yield or nitrogen utilizationefficiency under lower fertility.
 3. A vector comprising at least onepolynucleotide of claim
 1. 4. An expression cassette comprising at leastone polynucleotide of claim 1 operably linked to a promoter, wherein thepolynucleotide is in sense orientation.
 5. A host cell into which isintroduced at least one expression cassette of claim
 4. 6. The host cellof claim 5 that is a plant cell.
 7. A transgenic plant comprising atleast one expression cassette of claim
 4. 8. The transgenic plant ofclaim 7, wherein the plant is selected from the group consisting of:corn, soybean, sunflower, sorghum, canola, wheat, alfalfa, cotton, rice,barley and millet.
 9. A seed from the transgenic plant of claim
 7. 10.The seed of claim 9, wherein the seed is corn, soybean, sunflower,sorghum, canola, wheat, alfalfa, cotton, rice, barley or millet.
 11. Anisolated polypeptide selected from the group consisting of: a) anisolated polypeptide comprising any one of SEQ ID NOS: 7, 8, 9, 10, 11or 12 said polypeptide having NR activity; b) a polypeptide that is atleast 70% identical to the amino acid sequence of any of SEQ ID NOS: 7,8, 9, 10, 11 or 12 said polypeptide having NR activity; c) a polypeptidethat is encoded by a nucleic acid molecule comprising a nucleotidesequence that is at least 70% identical to any one of SEQ ID NOS: 1, 2,3, 4, 5 or 6 or a complement thereof, said polypeptide having NRactivity; d) a polypeptide that is encoded by a nucleic acid moleculethat hybridizes with a nucleic acid probe consisting of the nucleotidesequence of any of SEQ ID NOS: 1, 2 or 3, or a complement thereoffollowing at least one wash in 0.2×SSC at 55° C. for 20 minutes, saidpolypeptide having NR activity; e) a fragment comprising at least 200consecutive amino acids of any of SEQ ID NOS: 7, 8, 9, 10, 11, or 12,said polypeptide having NR activity.
 12. A recombinant expressioncassette comprising a polynucleotide operably linked to a promoter,wherein the polynucleotide encodes the polypeptide of claim
 11. 13. Atransformed host cell comprising the isolated polypeptide of claim 11.14. The host cell of claim 13, wherein the host cell is a transformedplant cell.
 15. The plant cell of claim 14, wherein the plant cell isselected from the group consisting of sorghum, maize, rice, wheat,soybean, sunflower, canola, alfalfa, barley and millet.
 16. Atransformed plant regenerated from the plant cell of claim
 14. 17. Theplant of claim 16, wherein the plant is sorghum, maize, rice, wheat,soybean, sunflower, canola, alfalfa, barley or millet.
 18. A transformedseed of the plant of claim
 16. 19. An isolated polypeptide encoded bythe polynucleotide of SEQ ID NO: 1, 2, 3, 4, 5 or
 6. 20. The isolatedpolypeptide of claim 11 wherein the expression of NR in a plant resultsin an increased yield in the plant as compared to a control plant,wherein the control plant that does not contain the polynucleotideencoding the NR.
 21. A method of modulating the level of nitratereductase (NR) protein in a plant cell, comprising: (a) transforming aplant cell with a NR polynucleotide operably linked to a promoter,wherein the polynucleotide is in sense orientation; and (b) expressingthe polynucleotide for a time sufficient to modulate the NR protein inthe plant cell.
 22. The method of claim 21, wherein the plant is corn,soybean, sunflower, sorghum, canola, wheat, alfalfa, cotton, rice,barley or millet.
 23. The method of claim 21, wherein NR protein isincreased.
 24. The method of claim 21, wherein NR protein is decreased.25. A method of modulating the level of nitrate reductase (NR) proteinin a plant, comprising: (a) stably transforming a plant cell with a NRpolynucleotide operably linked to a promoter, wherein the polynucleotideis in sense orientation; and (b) regenerating the transformed plant cellinto a transformed plant that expresses the NR polynucleotide in anamount sufficient to modulate the level of NR protein in the plant. 26.The method of claim 25, wherein the plant is corn, soybean, sunflower,sorghum, canola, wheat, alfalfa, cotton, rice, barley or millet.
 27. Themethod of claim 25, wherein NR protein is increased as compared to acontrol plant, wherein the control plant that does not contain thepolynucleotide encoding the NR.
 28. The method of claim 25, wherein NRprotein is decreased as compared to a control plant, wherein the controlplant that does not contain the polynucleotide encoding the NR.
 29. Amethod for increasing yield in a plant, said method comprising the stepsof: (a) introducing into plant cells a construct comprising apolynucleotide encoding a nitrate reductase (NR), wherein said NRpolynucleotide is operably linked to a promoter functional in plantcells to yield transformed plant cells, and wherein the NR encoding theNR protein is selected from the group consisting of: (1) apolynucleotide that encodes the polypeptide of SEQ ID NO: 7, 8, 9, 10,11 or 12; (2) a polynucleotide comprising the sequence set forth in theencoding region of SEQ ID NO: 7, 8, 9, 10, 11 or 12; and (3) apolynucleotide comprising at least 30 nucleotides in length whichhybridizes under moderate stringency conditions to a polynucleotide of(a) or (b), wherein the conditions include hybridization in 40 to 45%formamide, 1 M NaCl, 1% SDS at 37° C. and a wash in 0.5× to 1×SSC at 55to 60° C.; and (4) a polynucleotide having at least 70% sequenceidentity to SEQ ID NO: 1, 2, 3, 4, 5 or 6, wherein the % sequenceidentity is based on the entire encoding region and is determined byBLAST 2.0 under default parameters; and (5) an isolated polynucleotidedegenerate from any of (1) to (4) as a result of the genetic code; (6) apolynucleotide complimentary to a polynucleotide of any one of (1) to(5); (b) regenerating a transgenic plant from said transformed plantcells, wherein said NR is expressed in the cells of said transgenicplant at levels sufficient to maintain or increase yield in saidtransgenic plant.
 30. The method of claim 29, wherein increased yieldcomprises enhanced root growth, increased seed size, increased seedweight, the plant has seed with increased embryo size, increased leafsize, increased seedling vigor, enhanced silk emergence, increased earsize or chlorophyll content.
 31. The method of claim 29, wherein theplant is grown under limited nitrogen fertility.
 32. The method of claim31, wherein the nitrogen utilization efficiency of the plant isincreased.
 33. The method of claim 31, wherein NR activity of the NRpolynucleotide is increased compared to the activity of a NR endogenousto the plant.
 34. The method of claim 29, wherein the expression of theNR polynucleotide is driven by a phosphoenopyruvate decarboxylase (PEPC)promoter.
 35. The method of claim 29, wherein said polynucleotideencoding the NR is constitutively expressed.
 36. The method of claim 29,wherein said polynucleotide encoding the NR is inducibly expressed. 37.The method of claim 29, wherein said polynucleotide encoding the NR isexpressed in a cell-specific, tissue-specific, or organ-specific manner.38. The method of claim 29, wherein the plant is a dicotyledonous plant.39. The method of claim 29, wherein the plant is a monocotyledonousplant.
 40. The method of claim 29, wherein the yield of the plant iscompared to a control plant, wherein the control plant does not containthe polynucleotide encoding the NR.